Provide a way to change how the python executable is found #96

Job	Run time
test (ubuntu-latest, 26.3, false)	26s
test (ubuntu-latest, 27.2, false)	38s
test (ubuntu-latest, 28.2, false)	39s
test (ubuntu-latest, 29.3, false)	52s
test (macos-latest, 26.3, false)	1m 33s
test (macos-latest, 27.2, false)	2m 12s
test (macos-latest, 28.2, false)	1m 52s
test (macos-latest, 29.3, false)	1m 55s
test (windows-latest, 26.3, false)	2m 27s
test (windows-latest, 27.2, false)	1m 32s
test (windows-latest, 28.2, false)	57s
test (windows-latest, 29.3, false)	1m 51s
test (ubuntu-latest, snapshot, true)	32s
test (macos-latest, snapshot, true)	1m 50s
test (windows-latest, snapshot, true)	1m 14s
	20m 30s

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