[Github] Build CI Containers in Stacked PRs (llvm#149346)

Currently the pull_request event on the build CI container workflows are
restricted to main. This prevents building them on stacked PRs. This is
a bit annoying because we do not get the CI to test that everything is
working until all of the base PRs have landed and the target branch
becomes main.

Author: boomanaiden154

.github/workflows/build-ci-container-windows.yml

@@ -11,8 +11,6 @@ on:
       - .github/workflows/build-ci-container-windows.yml
       - '.github/workflows/containers/github-action-ci-windows/**'
   pull_request:
-    branches:
-      - main
     paths:
       - .github/workflows/build-ci-container-windows.yml
       - '.github/workflows/containers/github-action-ci-windows/**'

.github/workflows/build-ci-container.yml

@@ -11,8 +11,6 @@ on:
       - .github/workflows/build-ci-container.yml
       - '.github/workflows/containers/github-action-ci/**'
   pull_request:
-    branches:
-      - main
     paths:
       - .github/workflows/build-ci-container.yml
       - '.github/workflows/containers/github-action-ci/**'