Fix failing test_rlib_dep_reader_call for omit CARGO_HOME

[Felixoid]

Fixes #2303

If the CARGO_HOME is empty, but rustc is installed globally, it will be used in the compiler::rust::test::test_rlib_dep_reader_call.

Now, the test fails as:

---- compiler::rust::test::test_rlib_dep_reader_call stdout ----

thread 'compiler::rust::test::test_rlib_dep_reader_call' panicked at src/compiler/rust.rs:3321:33:
called `Result::unwrap()` on an `Err` value: No such file or directory (os error 2)

[sylvestre]

is it possible to add a test to make sure it doesn't regress? thanks

[codecov-commenter]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.05%. Comparing base (cd7dcd5) to head (7e3cbf2).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/compiler/rust.rs	40.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2527   +/-   ##
=======================================
  Coverage   71.04%   71.05%           
=======================================
  Files          64       64           
  Lines       35369    35393   +24     
=======================================
+ Hits        25128    25147   +19     
- Misses      10241    10246    +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Felixoid]

I don't know how to do it. The test is environment-dependent, so what should the regression test do?

Neither of rustc in $PATH nor $CARGO_home/bin/rustc looks mandatory to me. It's unclear how the regression can be caught.

[Felixoid]

I added the comment on why the code looks like this. It's the best I can think of.

Checking if rustc exists when CARGO_HOME is not set would fail for environments where there's no globally installed rust. And vise-versa as well, that's why this fallback is actually necessary.