bugfix: sarif: use absolute physical locations only

[woodruffw]

This changes the SARIF generation so that physicalLocations now always contains an absolute path to the input. This is a slight over-correction of the previous behavior, which would use a repo-relative path when possible and fall back on the filename component when it couldn't infer one.

In practice, the trick used by the previous behavior worked well for workflow path (which are always under .github/workflows but doesn't generalize for action paths, which can be anywhere. As a result there's no "anchor" to distinguish the repository prefix from the rest of the path.

This should have no negative effect on most users: local users will have their full paths in the SARIF instead, as will remote (GitHub Actions) users. In the latter case, GitHub Advanced Security should be able to "rebase" the paths correctly.

Fixes #452.

[woodruffw]

Did some more looking: GitHub recommends relative paths (reasonable), but it looks like they'll handle absolute paths in SARIFs just fine: upload-sarif will relativize using checkout_path, which defaults to ${{ github.workspace }}. That will be correct for 99.9% of users; certainly more correct than the previous behavior.

https://github.com/github/codeql-action/blob/c2c0a2908e95769d01b907f9930050ecb5cf050d/analyze/action.yml#L44-L47