Allow building against lingua-franca fork

.github/workflows/build.yml

@@ -4,15 +4,27 @@ on:
   workflow_call:
     inputs:
       nightly:
-        required: false
         type: boolean
+        description: Whether this is a nightly build
+        required: false
         default: false
       upload-artifacts:
         type: boolean
+        description: Whether to upload build artifacts
         required: false
         default: true
+      epoch-ref:
+        type: string
+        description: Ref of the epoch repository to check out
+        required: false
+        default: main
       lingua-franca-ref:
         type: string
+        description: Ref of the lingua-franca submodule to check out
+        required: false
+      lingua-franca-repo:
+        type: string
+        description: Full repository name of the form <owner>/<repo>
         required: false
     secrets:
       gpg-passphrase:
@@ -36,6 +48,7 @@ jobs:
         uses: actions/checkout@v3
         with:
           repository: lf-lang/epoch
+          ref: ${{ inputs.epoch-ref }}
           submodules: recursive
           fetch-depth: 0
       - uses: actions/setup-java@v3
@@ -45,13 +58,19 @@ jobs:
       - name: Check Maven/Java configuration
         run: mvn -version
         shell: bash
+      - name: Point submodule to given repo
+        run: |
+          git submodule set-url -- org.lflang/lingua-franca https://github.com/${{ inputs.lingua-franca-repo }}
+          git submodule update --init --recursive
+        shell: bash
+        if: ${{ inputs.lingua-franca-repo != '' }}
       - name: Update `lingua-franca` submodule
         run: |
           cd org.lflang/lingua-franca
           git fetch --all
           git checkout ${{ inputs.lingua-franca-ref }}
           git pull
-          git submodule update
+          git submodule update --recursive
         shell: bash
         if: ${{ inputs.lingua-franca-ref != '' }}
       - name: Configure Git for francabot