ci: apply IWYU suggestions to the source checkout, not the build tree

lint-tidy.sh ran fix_includes.py from build-ci/dashcore-*/src, the distdir
copy that configure and make use. It rewrote the includes of those copies,
so the "Run unit tests" step compiled the rewritten files and failed on the
includes IWYU dropped, and the git diff that should show the IWYU patch
printed nothing because the distdir is not tracked.

Upstream bitcoin#27012 runs fix_includes.py in the source checkout; the
backport changed that to the build directory. IWYU reports paths relative
to the build's src directory, which has the same layout as the checkout's
src, so run fix_includes.py from ${BASE_ROOT_DIR}/src as upstream does.

Moving the linters after the unit tests in build-src.yml is not enough for
pull requests: build.yml runs on pull_request_target, which takes the
workflow files from the base branch, so only the scripts come from the PR.

Author: knst

ci/dash/lint-tidy.sh

@@ -109,6 +109,6 @@ iwyu_tool.py \
   -Xiwyu --max_line_length=160 \
   2>&1 | tee "/tmp/iwyu_ci.out"
 
-cd "${BASE_ROOT_DIR}/build-ci/dashcore-${BUILD_TARGET}/src"
+cd "${BASE_ROOT_DIR}/src"
 fix_includes.py --nosafe_headers < /tmp/iwyu_ci.out
 git --no-pager diff