Update GitHub Actions workflows

[mheap]

Some workflows are configured to only run on the default branch.

Update them to use the default-branch macro (see https://github.blog/changelog/2020-07-22-github-actions-better-support-for-alternative-default-branch-names/)

[foolip]

A few other CI systems possibly worth detecting:

• Azure Pipelines
• CircleCI
• Cirrus CI
• Travis CI

I expect Travis CI in particular is pretty commonly used.