Copilot: use runner.temp for correct temp dir

.github/workflows/copilot-setup-steps.yml

@@ -17,9 +17,6 @@ jobs:
     permissions:
       contents: read
 
-    env:
-      COPILOT_BUILDTOOLS_DIR: /tmp/copilot-build-tools
-
     steps:
       - name: Checkout code including submodules
         uses: actions/checkout@v4
@@ -45,6 +42,9 @@ jobs:
           cache-dependency-path: syncthing/src/github.com/syncthing/syncthing/go.sum
 
       - name: Setup Android NDK
+        env:
+          COPILOT_BUILDTOOLS_DIR: '${{ runner.temp }}/copilot-build-tools'
+        shell: bash
         run: |
           ANDROID_NDK_VERSION=r28
           ANDROID_NDK_ZIP="android-ndk-${ANDROID_NDK_VERSION}-linux.zip"