one concurrency lock per branch

Author: myk002

.github/workflows/watch-df-steam.yml

@@ -10,7 +10,7 @@ jobs:
     if: github.repository == 'DFHack/dfhack'
     name: Check DF ${{ matrix.df_steam_branch }} branch
     runs-on: ubuntu-latest
-    concurrency: steampoll
+    concurrency: steampoll-${{ matrix.df_steam_branch }}
     strategy:
       fail-fast: false
       matrix: