Remove installPython from checks which should no longer need it

[mbg]

We added explicit setup-python steps to PR checks a while back because the default CodeQL CLI version at the time had a bug when used with the (then new) default Python version on Actions runners.

The default CodeQL CLI version should not contain the fix, so this is no longer necessary for any PR check other than multi-language-autodetect.yml which uses the defaultTestVersions and so includes some older CLI versions which predate the fix.

I think that the last CLI version without a fix would have been https://github.com/github/codeql-action/releases/tag/codeql-bundle-v2.23.3, so we could also (separately) look at making the remaining setup-python step conditional for versions below that, rather than !nightly-latest.

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• End-to-end tests - I am depending on PR checks (i.e. tests in pr-checks).

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Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.