Fix test coverage for Ubuntu 20.04

Author: cmcgee1024

.github/workflows/pull_request.yml

@@ -43,7 +43,7 @@ jobs:
     name: Test
     uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main
     with:
-      linux_os_versions: "[\"jammy\"]"
+      linux_os_versions: "[\"focal\"]"
       # We only care about the current stable release, because that's where we make our swiftly releases
       linux_exclude_swift_versions: "[{\"swift_version\": \"nightly-main\"},{\"swift_version\": \"nightly-6.0\"},{\"swift_version\": \"5.8\"},{\"swift_version\": \"5.9\"},{\"swift_version\": \"5.10\"}]"
       linux_pre_build_command: ./scripts/prep-gh-action.sh