Add an ENVIRONMENT option to add_halide_library

[alexreinking]

add_halide_library gains ENVIRONMENT VAR=value..., which sets environment variables for that library's generator invocations only (via cmake -E env). Entries must look like NAME=value.
The motivating use is HL_LLVM_ARGS: LLVM options are process-global, so this is the only way to give one library a flag such as -misched-regpressure=false without applying it to every library in the build.
Because HL_LLVM_ARGS can change codegen, the generator compile cache now includes it in the cache key; previously a cached result could be reused across different HL_LLVM_ARGS.
correctness_generator_cache checks that changing HL_LLVM_ARGS misses the cache (it fails without the key change). The new aot_generator_environment integration test passes an unknown LLVM option through ENVIRONMENT and checks that LLVM rejects it.

---

Authored by GitHub Copilot

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.13%. Comparing base (ee9f17a) to head (f522dad).

Files with missing lines	Patch %	Lines
src/Generator.cpp	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9545      +/-   ##
==========================================
+ Coverage   71.05%   71.13%   +0.07%     
==========================================
  Files         262      262              
  Lines       81084    81088       +4     
  Branches    19770    19770              
==========================================
+ Hits        57615    57679      +64     
+ Misses      17502    17480      -22     
+ Partials     5967     5929      -38     

☔ View full report in Codecov by Harness.
📢 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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.