Optimize _is_gnu_tar check

[DavidZbarsky-at]

Before:

After:

I have another version of this PR switching to the hermetic bsdtar toolchain (basically resolving it in the repo rule/module extension and passing it down in the same way), but this seems safer for now.

I also threw in some minor optimizations along the way while I was in the code.

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Changes are visible to end-users: no

Test plan

- Covered by existing test cases

[aspect-workflows]

Test

All tests were cache hits

193 tests (100.0%) were fully cached saving 42s.

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Test

e2e/bzlmod

All tests were cache hits

4 tests (100.0%) were fully cached saving 486ms.

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Test

e2e/gyp_no_install_script

All tests were cache hits

2 tests (100.0%) were fully cached saving 557ms.

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Test

e2e/js_image_oci

All tests were cache hits

1 test (100.0%) was fully cached saving 7s.

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Test

e2e/npm_link_package

All tests were cache hits

2 tests (100.0%) were fully cached saving 1s.

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Test

e2e/npm_link_package-esm

All tests were cache hits

2 tests (100.0%) were fully cached saving 923ms.

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Test

e2e/npm_translate_lock

All tests were cache hits

1 test (100.0%) was fully cached saving 73ms.

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Test

e2e/npm_translate_lock_empty

All tests were cache hits

1 test (100.0%) was fully cached saving 73ms.

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Test

e2e/npm_translate_lock_multi

All tests were cache hits

2 tests (100.0%) were fully cached saving 271ms.

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Test

e2e/npm_translate_lock_partial_clone

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1 test (100.0%) was fully cached saving 117ms.

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Test

e2e/npm_translate_lock_subdir_patch

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1 test (100.0%) was fully cached saving 251ms.

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Test

e2e/npm_translate_package_lock

All tests were cache hits

1 test (100.0%) was fully cached saving 73ms.

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Test

e2e/npm_translate_yarn_lock

All tests were cache hits

1 test (100.0%) was fully cached saving 73ms.

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Test

e2e/package_json_module

All tests were cache hits

1 test (100.0%) was fully cached saving 566ms.

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Test

e2e/pnpm_lockfiles

All tests were cache hits

28 tests (100.0%) were fully cached saving 10s.

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Test

e2e/pnpm_workspace

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8 tests (100.0%) were fully cached saving 3s.

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Test

e2e/pnpm_workspace_rerooted

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6 tests (100.0%) were fully cached saving 2s.

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Test

e2e/repo_mapping

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2 tests (100.0%) were fully cached saving 453ms.

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Test

e2e/rules_foo

All tests were cache hits

2 tests (100.0%) were fully cached saving 605ms.

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Test

e2e/vendored_node

All tests were cache hits

1 test (100.0%) was fully cached saving 187ms.

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