Since https://github.com/actions/runner-images/pull/11706,
https://github.com/actions/runner-images/pull/11707 and
https://github.com/actions/runner-images/pull/11737, ninja is
preinstalled in the Ubuntu, macOS and x86 Windows runner images.
Thanks to that, redundant installs of ninja on Unix were removed
in e20f0e3acd.
As noted in https://github.com/actions/partner-runner-images/issues/96,
ninja now also seems to be available on the Windows on ARM
images - thus remove redundant installs of ninja on Windows.