Add Amazon Linux 2023 to CI

Amazon Linux 2023 can't be added to `linux_os_versions` directly: it is
crossed with `linux_swift_versions`, and there is no
`swift:6.2-amazonlinux2023` image, so a 6.2 cell fails to pull. Run it
as a standalone job pinned to nightly-main instead. Coverage is
intentionally partial: nightly-main only, native test only. The static
Linux SDK build is omitted; it cross-compiles to musl, so the host OS
adds little coverage, and the nightly-main static-SDK cell additionally
hit a toolchain-snapshot skew.

Author: broken-circle

.github/workflows/pull_request.yml

@@ -12,7 +12,7 @@ jobs:
     name: Test
     uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main
     with:
-      linux_os_versions: '["bookworm", "noble", "jammy", "rhel-ubi9"]'
+      linux_os_versions: '["amazonlinux2023", "bookworm", "noble", "jammy", "rhel-ubi9"]'
       linux_swift_versions: '["6.2", "nightly-main"]'
       linux_pre_build_command: |
         if command -v apt-get >/dev/null 2>&1 ; then # bookworm, noble, jammy
@@ -24,12 +24,13 @@ jobs:
             apt-get update -y
             apt-get install -y procps
           fi
-        elif command -v dnf >/dev/null 2>&1 ; then # rhel-ubi9
+        elif command -v dnf >/dev/null 2>&1 ; then # amazonlinux2023, rhel-ubi9
           dnf update -y
 
           # Test dependencies
           dnf install -y procps
         fi
+      linux_exclude_swift_versions: '[{"swift_version": "6.2", "os_version": "amazonlinux2023"}]'
       linux_build_command: "swift test && swift test -c release && swift test --disable-default-traits"
       enable_freebsd_checks: true
       freebsd_build_command: "swift test && swift test -c release && swift test --disable-default-traits"