CI: Include Rust toolchain version in cargo cache keys to prevent cache poisoning

[RishavTiwari25]

This Pull Request fixes/closes #5181 .

It changes the following:

• Injected a Get rust_version step (rustc -V) in 6 workflow files (rust.yml, webassembly.yml, test262.yml, test262_release.yml, nightly_build.yml, and pull_request.yml) to capture the exact active Rust compiler version.
• Updated 15 actions/cache key strings to include -${{ steps.rust_version.outputs.version }}-, ensuring Cargo caches are strongly segregated by Rust compiler version, which prevents cross-version cache poisoning and silent linkage failures.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,126	50,126	0
Ignored	2,025	2,025	0
Failed	812	812	0
Panics	0	0	0
Conformance	94.64%	94.64%	0.00%

Tested main commit: be4cde434e03a9b3b117dc1f39ae394efffdfb54
Tested PR commit: 7ccbb00ddafb5c96dce81e8f7741e019d34aeb09
Compare commits: be4cde4...7ccbb00

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.80%. Comparing base (6ddc2b4) to head (7ccbb00).
⚠️ Report is 902 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5182       +/-   ##
===========================================
+ Coverage   47.24%   59.80%   +12.55%     
===========================================
  Files         476      582      +106     
  Lines       46892    63414    +16522     
===========================================
+ Hits        22154    37923    +15769     
- Misses      24738    25491      +753     

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[jedel1043]

Audited all uses of actions/checkout across the CI pipeline and upgraded any remaining outdated SHAs to the consistent actions/checkout@de0fac2 # v6 used elsewhere in the repo.

Deleted this part of the description since there were no updates related to this

[jedel1043]

I don't think we need this. Rustc will already evict the old cache if it detects that our dependencies were built with an older version of rustc, which effectively updates our cache to the latest rustc version.

If we merged this, we would instead increase our cache usage by a significant amount, since every time a new rustc version releases, it would create its own set of caches and leave the old set of caches there