Rebuild the windows docker image against the current ltsc [1]
of server 2019. This requires moving some of the msys setup
to the runner job cause it causes docker build to hang
Switch the job tags so they now use the 1809 runner, instead
of 1607.
Tweak the PATHs in the msys job so bash doesn't complain about
slashes..
Lastly, increase the timeout of the windows jobs, as msys2
installs its deps at runtime
[1] https://docs.microsoft.com/en-us/windows-server/get-started-19/servicing-channels-19
* Install git-lfs as its required now by gst-integration-suites
* Clone gst-build eache time to avoid dated gst-build checkouts
and overwritting .wrap files. Similar to !137
* Split the dockerfile and add a second run stage refresh the
powershell env inbetween calls
* Remove the msys2 workaround as its not needed anymore
