Add test for parallel compiler reproducible build

[aerooneqq]

Adds a test for reproducible binaries when using parallel compiler.

Confirmed with bisection and manual testing that #144722 fixed #140413, added test fails before commit (075ce31) from #144722.

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Regression in nightly-2025-08-14
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fetching https://static.rust-lang.org/dist/2025-08-13/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2025-08-13: 40 B / 40 B [============================================================================================================================================================================] 100.00 % 357.18 KB/s converted 2025-08-13 to 8e62bfd311791bfd9dca886abdfbab07ec54d8b4
fetching https://static.rust-lang.org/dist/2025-08-14/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2025-08-14: 40 B / 40 B [============================================================================================================================================================================] 100.00 % 123.86 KB/s converted 2025-08-14 to 3672a55b7cfd0a12e7097197b6242872473ffaa7
looking for regression commit between 2025-08-13 and 2025-08-14
fetching (via remote github) commits from max(8e62bfd311791bfd9dca886abdfbab07ec54d8b4, 2025-08-11) to 3672a55b7cfd0a12e7097197b6242872473ffaa7
ending github query because we found starting sha: 8e62bfd311791bfd9dca886abdfbab07ec54d8b4
get_commits_between returning commits, len: 7
  commit[0] 2025-08-12: Auto merge of #144678 - jdonszelmann:no-mangle-extern, r=bjorn3
  commit[1] 2025-08-12: Auto merge of #145295 - Kobzol:unify-stages, r=jieyouxu
  commit[2] 2025-08-13: Auto merge of #145093 - nikic:dead-on-return, r=nnethercote
  commit[3] 2025-08-13: Auto merge of #145334 - Kobzol:rollup-fs5a133, r=Kobzol
  commit[4] 2025-08-13: Auto merge of #144722 - ywxt:parallel-reproducibile, r=SparrowLii
  commit[5] 2025-08-13: Auto merge of #145298 - nikic:llvm21-rc3, r=cuviper
  commit[6] 2025-08-13: Auto merge of #145366 - GuillaumeGomez:rollup-v0a6v3u, r=GuillaumeGomez

Fixes #140413.

r? @petrochenkov