WIP: Add openra.
I've taken the openra definition from https://gitlab.com/pkill-9/guix-packages-free/ and tried to get it to work.
Previous versions relied on GeoIP, which is no longer an option since it requires a user account: https://github.com/OpenRA/OpenRA/issues/17508
Versions since late December 2019 don't need GeoIP, so that's fixed. Sadly, they require a rather up-to-date Mono toolchain, something like Mono 5.4 if I recall correctly.
Indeed, it heavily depends on msbuild
to build and install OpenRA.
So we need to update Mono in Guix. I gave it an unsuccessful shot in bug 39695.