GitLab doesn't wipe images entirely before reusing them across jobs.
It relies on the build scripts to not write outside the workdir.
However, we create symlinks outside of the workdir, which breaks in
weird ways.

Specifically, when ../../gstreamer/cerbero exists, `ln -sf` creates
../../gstreamer/cerbero/cerbero, which is wrong, but doesn't cause an
error. However, when the image is reused again after this,
../../gstreamer/cerbero/cerbero already exists, so `ln -sf` yields:

ln: ../../gstreamer/cerbero/cerbero: cannot overwrite directory

This is fixed in master after overhauling the scripts to never write
outside of workdir, but for 1.16 we need to hack it by not symlinking
if the target dir already exists.

Originally commited to the 1.16 branch in
!370

The proper fix will be backported from master, but let's fix this
error in the meantime so it doesn't roadblock the 1.18 stable releases

Part-of: <!374>