firefox: enable pull_request trigger for github workflow

Beside the manual trigger, also enable the pull requets trigger.

Differentiate between the manual and PR triggering when determining
the URLs and branches to be able to use both kind of triggers.

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>

Author: OldManYellsAtCloud

.github/workflows/yocto_matrix.yml

@@ -11,6 +11,11 @@ on:
         description: 'Branch to checkout for the workflow'
         required: true
         default: 'master'
+  pull_request:
+    branches:
+      - master
+    paths:
+      - 'meta-firefox/**'
 
 permissions:
   contents: read
@@ -40,8 +45,15 @@ jobs:
          mkdir -p /yocto/${{ matrix.yocto_version }}
          cd /yocto/${{ matrix.yocto_version }}
          rm -rf meta-browser meta-firefox-test
-         git clone $GITHUB_SERVER_URL/${{ github.event.inputs.repository }}/meta-browser
-         git -C meta-browser checkout ${{ github.event.inputs.branch }}
+         if [ "${{ github.event_name }}" = "pull_request" ]; then
+             GH_URL="$GITHUB_SERVER_URL/${{ github.event.pull_request.head.repo.full_name }}"
+             GH_REV="$GITHUB_HEAD_REF"
+         else
+             GH_URL="$GITHUB_SERVER_URL/${{ github.event.inputs.repository }}/meta-browser"
+             GH_REV="${{ github.event.inputs.branch }}"
+         fi
+         git clone $GH_URL
+         git -C meta-browser checkout $GH_REV
          # clone the test repo
          git clone https://github.com/OldManYellsAtCloud/meta-firefox-test.git
          ./meta-firefox-test/scripts/build.sh ${{ matrix.yocto_version}} ${{ matrix.arch }} ${{ matrix.ff_version }} ${{ matrix.libc_flavour}}