Expose `--python` as an environment variable (#3284) #8

Job	Run time
cargo test \| ubuntu	1s
cargo fmt	27s
cargo test \| macos	4m 1s
build binary \| linux	1s
build binary \| macos aarch64	1m 5s
build binary \| windows	1s
build binary \| macos x86_64	1s
cargo clippy \| ubuntu	44s
cargo test \| windows	1s
cargo clippy \| windows	1s
check windows trampoline	2m 55s
check system \| python on macos x86_64	0s
check system \| python on macos aarch64	15s
check system \| homebrew python on macos aarch64	17s
check cache \| macos aarch64	13s
ecosystem test \| ${{ matrix.repo }}	0s
check system \| python on debian	0s
check cache \| ubuntu	0s
check system \| python on fedora	0s
check system \| python on opensuse	0s
check system \| python on ubuntu	0s
check system \| alpine	0s
check system \| amazonlinux	0s
check system \| python3.12 via chocolatey	0s
check system \| python via pyenv	0s
check system \| pypy on ubuntu	0s
check system \| pyston	0s
check system \| python on rocky linux ${{ matrix.rocky-version }}	0s
check system \| python3.10 on windows	0s
check system \| python3.13 on windows	0s
check system \| python3.10 on windows x86	0s
check system \| conda${{ matrix.python-version }} on ${{ matrix.os }}	0s
check system \| embedded python3.10 on windows	0s
	10m 3s

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