Rollup merge of #141970 - onur-ozkan:skip-stage1-std, r=Kobzol

implement new `x` flag: `--skip-std-check-if-no-download-rustc`

One of our developers (``@RalfJung)`` [reported](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Surprising.20stages.20for.20check.20build.20after.20stage.20reorg/with/521925606)[#t-infra/bootstrap > Surprising stages for check build after stage reorg](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Surprising.20stages.20for.20check.20build.20after.20stage.20reorg/with/521925606) that working on both the compiler and the library simultaneously with RA enabled is extremely difficult because checking library creates a heavy load on machines (by building stage1 compiler) on each modification. `--skip-std-check-if-no-download-rustc` flag is intended to reduce this heavy load on their IDE integration as much as possible.

Fixes: rust-lang/rust#141955

Author: matthiaskrgr