Automerge: [Github][libcxx] Run Container Build Workflow on Stacked PRs (#150683)

Currently the container build workflow does not run on stacked PRs that
do not have a branch target of main because of how the workflow is
configured. This makes the workflow better confirm to the LLVM CI best
practices around Github workflows:

https://llvm.org/docs/CIBestPractices.html#ensuring-workflows-run-on-the-correct-events

Author: boomanaiden154