generate_board_targets: explicitly specify build containers

in order to have the right container version for each toolchain
currently for the metadata deployment GitHub actions build.

.github/workflows/deploy_all.yml

@@ -24,7 +24,7 @@ jobs:
     needs: enumerate_targets
     strategy:
       matrix: ${{fromJson(needs.enumerate_targets.outputs.matrix)}}
-    container: px4io/px4-dev-${{ matrix.container }}:2021-09-08
+    container: ${{ matrix.container }}
     steps:
     - uses: actions/checkout@v1
       with: