This brings some workflow improvements and fixes from the `cmake` branch
to `main`, as well as some unique fixes to keep gclient working, so that
we can continue to accept contributions in `main` until the `cmake`
merge is ready.
- Fix docs build in GitHub Actions (from `cmake` branch)
- Cancel workflow when a PR is updated (from `cmake` branch)
- Fix docker failures caused by running as root (from `cmake` branch)
- Work around exception in depot_tools on Windows
- Use Windows 2019 images in GitHub Actions for compatibility with gyp
- Remove Docker build on ArchLinux, which no longer supports python2 at
all
- (NOTE: The `cmake` branch is still building on ArchLinux. Docker
builds for Arch will be restored to the `main` branch when the `cmake`
branch is finally merged to `main`.)