python: rework how we compute the "command" property (#46850) #92

Job	Run time
changes	29s
prechecks / bootstrap-dev-rhel8	3m 59s
prechecks / style	58s
prechecks / import-check	2m 44s
prechecks / validate	39s
prechecks / audit / package-audits (windows-latest, powershell Invoke-Expression -Command "./share/spack/qa/windows_t...	4m 49s
prechecks / audit / package-audits (ubuntu-latest, bash)	8m 7s
prechecks / audit / package-audits (macos-latest, bash)	5m 9s
all-prechecks	1s
unit-tests	0s
bootstrap	0s
coverage	0s
all	0s
	26m 55s

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