Skip matrix jobs more cleanly (take 2)

Author: jakepetroules

.github/workflows/swift_package_test.yml

@@ -296,7 +296,6 @@ jobs:
 
   linux-build:
     name: Linux (${{ matrix.swift_version }} - ${{ matrix.os_version }})
-    if: ${{ inputs.enable_linux_checks }}
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
@@ -305,6 +304,8 @@ jobs:
         os_version: ${{ fromJson(inputs.linux_os_versions) }}
         exclude:
           - ${{ fromJson(inputs.linux_exclude_swift_versions) }}
+          - ${{ fromJson(inputs.enable_linux_checks ? '[]' : inputs.linux_swift_versions) }}
+          - ${{ fromJson(inputs.enable_linux_checks ? '[]' : inputs.linux_os_versions) }}
     container:
       image: ${{ (contains(matrix.swift_version, 'nightly') && 'swiftlang/swift') || 'swift' }}:${{ matrix.swift_version }}-${{ matrix.os_version }}
     steps: