Run on all commits to `master` and PR branches.
Aim for consistency re: GHA workflows for macOS and Linux.
Also switch from mingw64 to ucrt64 and drop use of sudo, etc. that aren't
relevant on Windows + mingw-w64-ucrt-x86_64.
Add README badge for the GHA workflow for Windows - let's make the "red light"
turn into a "green light"!
Also split out the build of libqrcodegen on macOS into a separate step, and
make the workflows for Ubuntu and macOS as consistent as possible; may require
more changes depending on what passes and doesn't pass on macOS.
The GHA workflow for Windows will be revised in a separate PR.
