Simplify flang preference helpers

[t-kalinowski]

Draft stacked on #110.

Removes test-only which and system2 arguments from the flang probe/compiler helper signatures. Tests now mock Sys.which() and system2() directly.

Testing:

• devtools::test()
• rcmdcheck::rcmdcheck(error_on = "warning")

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.05%. Comparing base (fa49565) to head (b682b91).

Files with missing lines	Patch %	Lines
R/compiler.R	90.90%	1 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##           cache-compiler-config     #112      +/-   ##
=========================================================
- Coverage                  93.11%   93.05%   -0.07%     
=========================================================
  Files                         28       28              
  Lines                       6015     6006       -9     
=========================================================
- Hits                        5601     5589      -12     
- Misses                       414      417       +3     

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