Add warning and update PR checks for Swift on Linux

[angelapwen]

We are deprecating Swift support on Linux for code scanning (changenote in #2403).

This PR logs a warning for users in the init Action if it's determined that Swift on Linux is being run.

This change also updates the manual build script build.sh which we also use in internal repositories for testing: the swift command will only be run on MacOS rather than on both MacOS and Linux.

As a result, we can also update our PR checks that only test on Linux, to remove the setup-swift Action step as it is no longer needed; and other checks that look for the existence of Swift databases on Linux.

I've removed Swift analysis using a custom build command check on Linux images from required checks for main, releases/v3, and releases/v2 already as well.

Merge / deployment checklist

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

[github-actions[bot]]

Pushed a commit to rebuild the Action. Please mark the PR as ready for review to trigger PR checks.

[angelapwen]

Huh.. not sure why the Unit Test workflow isn't being picked up. I'll push an empty commit to try to force it to run.