Make it possible to control revision at workflow dispatch

.github/workflows/build.yml

@@ -1,7 +1,11 @@
 name: build-native
 on:
   workflow_dispatch:
-    inputs: {}
+    inputs:
+      rev:
+        description: 'libsdl-org/SDL commit to build'
+        required: false
+        default: 'master'
 #  schedule:
 #    - cron: '0 0 * * *'
 env:
@@ -69,6 +73,7 @@ jobs:
     - uses: actions/checkout@v2
       with:
         repository: libsdl-org/SDL
+        ref: ${{ github.event.inputs.rev }}
     - uses: actions/checkout@v2
       with:
         path: 'SDL2-CS'