CI: Install Swift toolchain preserving its usr/bin layout

Flattening the tarball into /usr/local makes SwiftPM derive a bogus
toolchain root, so the swiftbuild build system cannot find swiftc.

Author: kateinoigakukun

.github/actions/install-swift/action.yml

@@ -32,6 +32,12 @@ runs:
         zlib1g-dev
         curl
 
+    # Keep the toolchain's `<root>/usr/bin` layout intact. Flattening it into
+    # /usr/local breaks toolchain discovery in the swiftbuild build system,
+    # which derives the toolchain root by stripping `usr/bin` from `swiftc`.
     - name: Install Swift
       shell: bash
-      run: curl -fL ${{ inputs.download-url }} | sudo tar xfz - --strip-components=2 -C /usr/local
+      run: |
+        sudo mkdir -p /opt/swift
+        curl -fL ${{ inputs.download-url }} | sudo tar xfz - --strip-components=1 -C /opt/swift
+        echo "/opt/swift/usr/bin" >> "$GITHUB_PATH"