GitHub Actions: build stage 2 of MXE image for correct branch

When triggered by a push event, the checkout action checks out the correct branch.
But without this change, the stage 2 action would always run on master.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

.github/workflows/mxe-dockerimage-stage1.yml

@@ -46,4 +46,4 @@ jobs:
                     -H "Accept: application/vnd.github.everest-preview+json" \
                     -H "Content-Type: application/json" \
                     https://api.github.com/repos/subsurface-divelog/subsurface/dispatches \
-                    --data '{"event_type": "${{ env.VERSION }}" }'
+                    --data '{"event_type": "${{ env.VERSION }}", "client_payload": { "target_branch": "${{ github.ref }}" } }'

.github/workflows/mxe-dockerimage-stage2.yml

@@ -12,6 +12,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v1
+      with:
+        ref: ${{ github.event.client_payload.target_branch }}
 
       # Grab the version from the event name that we were triggered by and add ".stage1" to find the docker image to start FROM
       # And create the NAME of the final docker image