Only check `packageManager` if it actually specifies `pnpm` #289

Job	Run time
Test with standalone (ubuntu-latest, true)	19s
Test with run_install (null, ubuntu-latest)	8s
Test with standalone (ubuntu-latest, false)	7s
Test with default inputs (4.11.1, ubuntu-latest)	10s
Test with run_install (empty object, ubuntu-latest)	13s
Test with standalone (macos-latest, true)	10s
Test with dest (4.11.1, ubuntu-latest)	9s
Test with default inputs (4.11.1, macos-latest)	10s
Test with run_install (recursive, ubuntu-latest)	11s
Test with standalone (macos-latest, false)	6s
Test with dest (4.11.1, macos-latest)	11s
Test with default inputs (4.11.1, windows-latest)	36s
Test with run_install (global, ubuntu-latest)	14s
Test with dest (4.11.1, windows-latest)	51s
Test with standalone (windows-latest, true)	34s
Test with run_install (array, ubuntu-latest)	16s
Test with standalone (windows-latest, false)	23s
Test with run_install (null, macos-latest)	10s
Test with run_install (empty object, macos-latest)	14s
Test with run_install (recursive, macos-latest)	13s
Test with run_install (global, macos-latest)	15s
Test with run_install (array, macos-latest)	19s
Test with run_install (null, windows-latest)	43s
Test with run_install (empty object, windows-latest)	36s
Test with run_install (recursive, windows-latest)	50s
Test with run_install (global, windows-latest)	50s
Test with run_install (array, windows-latest)	44s
	9m 42s

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