mingw: Pull opusfile source from gitlab.

Update the build environment container to use the gitlab.xiph.org
url instead of the no-longer-available git.xiph.org.
3 jobs for mingw in 1 minute (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #10456
docker
autotools

00:01:00

passed #10458
docker
doc

00:00:25

passed #10457
docker
makefile

00:00:23