Add warning and update PR checks for Swift on Linux (#2399)

* PR checks: Only run Swift build command on MacOS
* PR checks: update to only test Swift on MacOS
* Log warning if workflow is running Swift on Ubuntu
---------

Co-authored-by: Andrew Eisenberg <aeisenberg@github.com>

.github/workflows/__swift-custom-build.yml

@@ -29,16 +29,10 @@ jobs:
         include:
           - os: macos-latest
             version: linked
-          - os: ubuntu-latest
-            version: linked
           - os: macos-latest
             version: default
-          - os: ubuntu-latest
-            version: default
           - os: macos-latest
             version: nightly-latest
-          - os: ubuntu-latest
-            version: nightly-latest
     name: Swift analysis using a custom build command
     permissions:
       contents: read