Add nightly releases for lts-3.3 branch (#19987)

Closes #19079

Author: Kordyjan

.github/workflows/ci.yaml

@@ -613,6 +613,9 @@ jobs:
         - ${{ github.workspace }}/../../cache/sbt:/root/.sbt
         - ${{ github.workspace }}/../../cache/ivy:/root/.ivy2/cache
         - ${{ github.workspace }}/../../cache/general:/root/.cache
+    strategy:
+      matrix:
+        branch: [main, lts-3.3]
     needs: [test_non_bootstrapped, test, mima, community_build_a, community_build_b, community_build_c, test_sbt, test_java8]
     if: "(github.event_name == 'schedule' || github.event_name == 'workflow_dispatch') && github.repository == 'scala/scala3'"
     env:
@@ -641,6 +644,8 @@ jobs:
 
       - name: Git Checkout
         uses: actions/checkout@v4
+        with:
+          ref: ${{ matrix.branch }}
 
       - name: Add SBT proxy repositories
         run: cp -vf .github/workflows/repositories /root/.sbt/ ; true