Merge Cygwin CI workflow parts

[jmid]

This PR merges the two GitHub actions workflow parts for Cygwin into a single one.

Initially, the split was done out of necessity in #305 and #313 as the Cygwin port initially was too slow to complete within the 6h timeout!

Since then we have seen general OCaml (GC) improvements, meaning that the 2-part Cygwin CI workflow would complete in about 2x45m, and thus opening up for a workflow merge incl. saving time on storing and restoring the state.

Finally I offer two data points from a trial run of this branch:

• 5.1.1~rc1 completed in 1h16m
• trunk completed in 1h11m

[jmid]

Cygwin-wise, the 4 merged workflows completed 69m, 76m, 79m, and 84m, thus confirming that merging should be OK.
We may even consider lowering the time out at some point...

With the removal of the bytes dependency in qcheck-core.0.21.3 we had a record low 12m run on Linux testing 5.1 - and 12 complete GA runs of <=20m! 🎉

CI summary:

• Bytecode trunk triggered a segfault in Lin Out_channel test with Domain [ocaml5-issue] Segfault in Lin Out_channel test in 32-bit and bytecode mode #412
• Linux 5.1 debug failed an assertion in threadomain [ocaml5-issue] Assertion failure on threadomain under debug runtime #404
• MinGW bytecode timed out during threadomain [ocaml5-issue] Windows failures on threadomain #203
• freebsd-amd64-5.0 failed with an Internal error

Out of 59 workflows 4 failed with 3 genuine issues and 1 CI issue.

[shym]

LGTM!
And it’s really nice to be able to go back to a single job for those workflows, juggling with the timeout was unnecessarily complex.
Little suggestion: if we drop all that two-job logic on the .yml side, we might just as well as drop the skipnextjob lines on the dune side: shym@e7d04de
What do you think?

[shym]

I updated the PR according to my suggestion :-)

[jmid]

Of the 3 Cygwin runs 3 completed in 80m, 87m, 127m and 1 timed out after 6h because of a deadlock.

CI summary:

• Cygwin 5.1 aborted during the Dynlink test [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307
• Cygwin trunk timed out during the Lin Dynlink test [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307
• Cygwin trunk aborted with Fatal error: Failed to create domain during domain_spawntree [ocaml5-issue] Windows trunk bytecode domain_spawntree crash or deadlock #354
• Linux 5.1 debug triggered an assertion failure during thread_joingraph [ocaml5-issue] Abort / crash on thread_joingraph and thread_createtree under debug runtime #353
• Linux 5.1 debug triggered an assertion failure during thread_createtree [ocaml5-issue] Abort / crash on thread_joingraph and thread_createtree under debug runtime #353
• Linux trunk debug triggered multiple assertion failures:
  • thread_joingraph and the Lin Stack test triggered assertion failure in memprof.c Assertion failure triggered in runtime/memprof.c #422
  • the Lin Queue, Lin Bytes, Lin.Internal ref int test with Thread, and Lin.Internal CList int test with Thread all triggered assertion failures in domain.c [ocaml5-issue] Assertion failure on threadomain under debug runtime #404
• Linux trunk debug triggered multiple assertion failures:
  • threadomain, Lin Queue, Lin Stack, Lin Bytes, and lin_internal_tests_thread_conclist all triggered an assertion failure in memprof.c Assertion failure triggered in runtime/memprof.c #422
  • stm_tests_thread_ref and lin_internal_tests_thread_ref triggered an assertion failure in domain.c [ocaml5-issue] Assertion failure on threadomain under debug runtime #404
• MinGW 5.1 triggered a crash in threadomain [ocaml5-issue] Windows failures on threadomain #203
• MinGW bytecode trunk triggered a fatal error in domain_spawntree [ocaml5-issue] Windows trunk bytecode domain_spawntree crash or deadlock #354
• MinGW trunk triggered a fatal error in domain_spawntree [ocaml5-issue] Windows trunk bytecode domain_spawntree crash or deadlock #354
• macos-arm64-5.0 failed with Internal error
• freebsd-amd64-5.0 failed with Internal error
• freebsd-amd64-5.1 failed to find a Lin Out_channel test with Domain counterexample Lin Out_channel test fails to trigger on FreeBSD #401

Out of 59 workflows 13 failed, with 10 triggering genuine errors, 1 false alarm, and 2 internal CI failures

[jmid]

CI summary for merge to main:

• Cygwin trunk aborted during domain_spawntree with Failed to create domain [ocaml5-issue] Windows trunk bytecode domain_spawntree crash or deadlock #354
• Linux trunk debug triggered numerous assertion failures in runtime/domain.c Assertion failures in runtime/domain.c on trunk #424
• linux-arm64-5.2 (failed: Cancelled) timed out during thread_createtree - with Atomic [ocaml5-issue] Deadlock in 'thread_createtree - with Atomic' on Linux ARM64 #425
• macos-arm64-5.0 failed with Internal error (because of missing 5.0 images)
• freebsd-amd64-5.0 failed with Internal error (because of missing 5.0 images)

Out of 36 workflows 5 failed with 3 genuine issues and 2 CI setup-related failures