Use shutil when looking for executables on the host system

[mkrasnitski]

Description

The implementation of the which function only checks for the executable flag on paths, which means it can return a directory (at least on POSIX), causing weird permission errors down the line. It should instead defer to shutil.which; that function contains a platform-agnostic implementation that should hopefully match the behavior of the GNU which utility.

Checklist

• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.

[github-actions]

🤖 Coverage update for f053502 🟢

	Old	New
Commit	ba70548	f053502
Score	71.2773%	71.2773% (0)

[Grazfather]

Good call. Looks good. Have a tiny rec to simplify

[github-actions]

🤖 Coverage update for a00d9f1 🟢

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Commit	ba70548	a00d9f1
Score	71.2773%	71.2773% (0)

[github-actions]

🤖 Coverage update for 13ff22c 🟢

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Commit	ba70548	13ff22c
Score	71.2773%	71.2773% (0)