Cleanup environment variable usage

[justsmth]

Issues:

Addresses:

• aws-lc-sys add support for custom effective_target mappings #765
• FIPS build fails with cmake 4.0.0 on MacOS #764

Description of changes:

• Improvements/fixes for how a few CI tests were setting up environment variables.
• Fix MacOS FIPS build for CMake 4.0.
• Support, and give preference to, target-specific environment variables.
• Support an AWS_LC_SYS_EFFECTIVE_TARGET environment variable.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.90%. Comparing base (c358484) to head (c9d0bef).
Report is 200 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #763      +/-   ##
==========================================
- Coverage   95.80%   92.90%   -2.90%     
==========================================
  Files          61       71      +10     
  Lines        8143     9717    +1574     
  Branches        0     9717    +9717     
==========================================
+ Hits         7801     9028    +1227     
- Misses        342      421      +79     
- Partials        0      268     +268     

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[sunjay]

I have tested this fix locally using my team's internal repo and confirmed that it resolves the FIPS build issues for cmake 4.0.

After making the change below to my Cargo.toml file, I was able to run cargo build successfully again.

[patch.crates-io]
aws-lc-fips-sys = { git = "https://github.com/justsmth/aws-lc-rs.git", branch = "cleanup-ci-env-vars" }

Thanks @justsmth! Hope this can get merged and published soon!