games/openra: unbreak fetch after r479607

$ make distclean
$ make checksum MASTER_SITE_BACKUP=
[...]

> stylecop.msbuild.4.7.49.0.nupkg doesn't seem to exist in /distfiles/.

> Attempting to fetch https://api.nuget.org/v3-flatcontainer/stylecop.msbuild/4.7.49.0/stylecop.msbuild.4.7.49.0.nupkg

fetch: https://api.nuget.org/v3-flatcontainer/stylecop.msbuild/4.7.49.0/stylecop.msbuild.4.7.49.0.nupkg: Not Found

> Couldn't fetch it - please try to retrieve this

> port manually into /distfiles/ and try again.

• Error code 1