test(build): support build cache inside tests via env var

[geertj]

Checking GCSFUSE_USE_BUILD_CACHE environment variable in buildBinaries. If it is set to "true", we skip creating temporary, isolated GOPATH and GOCACHE folders and compile in standard module mode.

This keeps GCSFuse building cleanly/isolated by default, but allows unit tests to set GCSFUSE_USE_BUILD_CACHE to speed up TestVersion execution time from ~24s to ~3.6s.

TAG=agy
CONV=8c7b5ab6-91ae-4a84-911e-1788917d5ab8

[gemini-code-assist]

Summary of Changes

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This pull request optimizes the build process for gcsfuse by providing an opt-in mechanism to use the standard Go build cache. By allowing the build system to skip the creation of temporary, isolated directories when requested, the change improves developer productivity and reduces CI test execution times.

Highlights

• Build Cache Support: Introduced the GCSFUSE_USE_BUILD_CACHE environment variable to allow bypassing isolated GOPATH and GOCACHE creation during builds.
• Performance Optimization: Enabled build caching in unit tests, significantly reducing the execution time of TestVersion from approximately 24 seconds to 3.6 seconds.

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[gemini-code-assist]

Code Review

This pull request introduces build cache support for gcsfuse builds by checking the GCSFUSE_USE_BUILD_CACHE environment variable. When enabled, it skips creating temporary GOPATH and GOCACHE directories, which speeds up the build process and is also utilized in tests. The review feedback suggests improving the robustness of the environment variable check by supporting case-insensitive values and alternative truthy representations like '1'.