ci: take an optional shell parameter

Allow the build matrix to specify its shell, in case we provide one in
the setup (eg, Cygwin).

.github/actions/run-build/action.yml

@@ -5,14 +5,19 @@ description: Run a build step in a container or directly on the Actions runner
 inputs:
   command:
     description: Command to run
-    required: true
     type: string
+    required: true
   container:
     description: Optional container to run in
     type: string
   container-version:
     description: Version of the container to run
     type: string
+  shell:
+    description: Shell to use
+    type: string
+    required: true
+    default: 'bash'
 
 runs:
   using: 'composite'
@@ -42,4 +47,4 @@ runs:
       else
         ${{ inputs.command }}
       fi
-    shell: bash
+    shell: ${{ inputs.shell != '' && inputs.shell || 'bash' }}

.github/workflows/main.yml

@@ -241,12 +241,14 @@ jobs:
         command: cd build && ../source/ci/build.sh
         container: ${{ matrix.platform.container.name }}
         container-version: ${{ env.docker-registry-container-sha }}
+        shell: ${{ matrix.platform.shell }}
     - name: Test
       uses: ./source/.github/actions/run-build
       with:
         command: cd build && ../source/ci/test.sh
         container: ${{ matrix.platform.container.name }}
         container-version: ${{ env.docker-registry-container-sha }}
+        shell: ${{ matrix.platform.shell }}
     - name: Upload test results
       uses: actions/upload-artifact@v3
       if: success() || failure()