Fix salt-ssh not detecting platform-python as a valid interpreter on EL8

[lastmikoi]

What does this PR do?

This PR adds an additional (this time absolute) python path to salt-ssh's /bin/sh shim, to support deployments where the available interpreter is EL's platform-python (and thus mimic the salt RPM package behaviour).

This is a naive fix, I've originally thought about being able to use --python3-bin to steer salt-ssh into using the proper remote python, thus leaving the platform-python shenanigans to the user and their configuration choices, but that argument doesn't seem to be actually used by salt-ssh and my knowledge of the codebase isn't good enough to implement that change.

What issues does this PR fix or reference?

Fixes: #62235

Previous Behavior

salt-ssh would complain about Python 3 not being installed on a remote EL8 target even if salt is installed on the target.

New Behavior

salt-ssh finds the /usr/libexec/platform-python interpreter as an ultimate fallback to other more common python paths.

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated -- The test suite doesn't appear to support testing this specific usecase

Commits signed with GPG?

Yes

[welcome]

Congratulations on your first PR being merged! 🎉