set up scheduled nightly build for 1.0.x branch

[pjfanning]

• we could add a yml file in main branch that checks out ref: 1.0.x - we do this for a few jobs already
• see top answer on https://stackoverflow.com/questions/58798886/github-actions-schedule-operation-on-branch as to why we can't use the existing https://github.com/apache/incubator-pekko/blob/main/.github/workflows/nightly-builds.yml - its schedule applies only to the main branch
• we could possibly use https://github.com/apache/incubator-pekko/blob/main/.github/workflows/nightly-builds.yml but add a matrix element called checkoutBranch set to [main, 1.0.x] and change the checkout steps to apply ref: ${{ matrix.checkoutBranch }}
• in mean time, you can still run the tests manually at https://github.com/apache/incubator-pekko/actions/workflows/nightly-builds.yml and choose the 1.0.x branch

[pjfanning]

this is done