CI: use NFS cache for Yocto on self-hosted runner

Speed up the pipeline by reusing the Yocto download and sstate
cache directories on the self-hosted runner via NFS mounts.

Mostly for `devtool modify busybox` which needs to build quilt-native
and a few other recipes.

Author: deribaucourt

.github/workflows/main.yml

@@ -99,6 +99,11 @@ jobs:
 
     - name: Test Project
       run: npm run test
+      env:
+        # NFS Yocto cache directories on the self-hosted runner
+        DL_DIR: /var/cache/yocto/yocto-dldir
+        SSTATE_DIR: /var/cache/yocto/yocto-sstate
+        BB_ENV_PASSTHROUGH_ADDITIONS: "DL_DIR SSTATE_DIR"
 
     - name: Build the VSIX
       run: npm run package