CI: switch to using Linux for testing

As the GitHub virtual environment now has the swift toolchain, use
that to build swift-lint and run the tests.

.github/workflows/test.yml

@@ -81,18 +81,21 @@ jobs:
           cd ./test/linters/projects/rubocop/
           bundle install
 
+      # Swift (only on Linux)
+
+      - name: install swift-format
+        if: startsWith(matrix.os, 'ubuntu')
+        run: |
+          git clone -b swift-5.2-branch git://github.com/apple/swift-format
+          cd swift-format
+          swift build -c release
+          echo "::add-path::${PWD}/.build/release"
+
       # Swift (only on macOS)
 
       - name: Install Swift dependencies
         if: startsWith(matrix.os, 'macos')
         run: |
-          # Because swift-format requires a matching toolchain, use the OSS
-          # toolchain to ensure that we are using a known revision of the
-          # toolchain which should enable the use of swift-format for testing.
-          curl -OL https://swift.org/builds/swift-5.2.1-release/xcode/swift-5.2.1-RELEASE/swift-5.2.1-RELEASE-osx.pkg
-          sudo installer -verbose -pkg "${PWD}/swift-5.2.1-RELEASE-osx.pkg" -target /
-          export TOOLCHAINS=swift
-
           brew update
           brew install mint
 
@@ -102,9 +105,6 @@ jobs:
           cd ../swiftlint/
           mint bootstrap --link
 
-          cd ../swift-format
-          brew install swift-format
-
       # Tests
 
       - name: Run tests

test/linters/linters.test.js

@@ -32,9 +32,13 @@ const linterParams = [
 	xoParams,
 ];
 
-// Only run Swift tests on macOS
+if (process.platform === "linux") {
+	linterParams.push(AppleSwiftFormatParams);
+}
+
+// Only run some Swift tests on macOS
 if (process.platform === "darwin") {
-	linterParams.push(AppleSwiftFormatParams, swiftformatParams, swiftlintParams);
+	linterParams.push(swiftformatParams, swiftlintParams);
 }
 
 // Copy linter test projects into temporary directory