PR checks: small fixups to prepare-action

[angelapwen]

While working on a version of this for testing on runner images, I noticed some improvements to be made:

• the action now expects stable CLI version inputs as stable-vX.Y.Z rather than with dates. Our current PR checks already use this format but the documentation for the input needed to be updated.
• the nightly version format also should expect nightly-YYYYMMDD rather than with dashes
• the nightly bundle URL formatting has been corrected to remove the manual suffix — eg. the current nightly URLs look like: https://github.com/dsp-testing/codeql-cli-nightlies/releases/download/codeql-bundle-20240823/codeql-bundle-linux64.tar.gz. We haven't caught this because none of our PR checks use the dated nightly bundles.
• when the version is linked or default we return the appropriate tools input early.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.