Use tagged version of DiscordRPC (#379)

This way builds are more reproducible as specific versions of
NorthstarDiscordRPC can be set.

.github/workflows/build.yml

@@ -21,6 +21,7 @@ jobs:
         uses: actions/checkout@v3
         with:
           repository: R2Northstar/NorthstarDiscordRPC
+          ref: 'v1'
           path: discord-plugin
       - name: Checkout launcher repository
         uses: actions/checkout@v3