Use the parallel bundle build on the CI

.github/workflows/test.yml

@@ -107,11 +107,14 @@ jobs:
         # Instead of hardcoding which targets to build and package, we'll
         # package everything that's got en entry in the `bundler.toml` file
         run: |
-          packages=$(cargo xtask known-packages)
-          for package in $packages; do
-            cargo xtask bundle "$package" --release
+          # Building can be sped up by specifying all packages in one go
+          package_args=()
+          for package in $(cargo xtask known-packages); do
+            package_args+=("-p" "$package")
           done
 
+          cargo xtask bundle "${package_args[@]}" --release
+
       - name: Determine build archive name
         run: |
           echo "ARCHIVE_NAME=nih-plugs-$(git describe --always)-${{ matrix.os }}" >> "$GITHUB_ENV"