Re-enable CLANG64 environment. Add CLANGARM64. Don't add MINGW64
to slightly reduce the number of runner VMs needed.
Install the required packages using the setup-msys2 action instead
of running the commands separately.
Test Autotools and CMake in the same job to reduce the number of VMs.
This doesn't slow it down too much because the msys2-setup step is
needed by both. However, do only the full builds on ARM64 because
those runners seem to be slower.
Test fewer build configurations. The point of testing on MSYS2 is to
catch Windows-related issues. It should be enough that the more unusual
build configurations are tested in ci.yml.
Run the build commands directly instead of using ci_build.bash. This
makes it easier to see what commands are run even if it is a little
more verbose now.
Run the workflow automatically when commit are pushed to master.
With the fewer build variants it's not too slow.
