CI housekeeping

[sajith]

• Separate code signing checks and code formatting checks. They should be triggered by different things, and mixing them up can be a source of confusion.
• Use a custom comment in code signing warnings, since FABRIC requires GPG-signed commits.
• Use current versions of actions (reason: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/).
• Use current stable Python release where possible.
• Use coverallsapp action to send coverage, not Python coveralls. The former appears to work more reliably.
• Add Python 3.12 to test matrix. (Edit: we can't do this yet.)
• Run unit tests on macos13 (Intel) and macos-14 (ARM) runners.
• Enable some workflows to be triggered manually.

[coveralls]

Pull Request Test Coverage Report for Build 9322486472

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-3.6%) to 18.308%

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Totals
Change from base Build 9303430979:	-3.6%
Covered Lines:	1165
Relevant Lines:	4703

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💛 - Coveralls

[sajith]

Here's an example of a comment left by code signing check: https://github.com/sajith/fabrictestbed-extensions/pull/2#issuecomment-2142763146

The default message has hints about signing using ssh and 1Password; but since FABRIC wants gpg-signed commits, I customized that message.