cerbero: Don't relocate deps cache when targeting Windows
cerbero: Don't relocate deps cache when targeting Windows

The dependency cache is already relocatable on Windows because the
final artifacts are fully relocatable too. It was also failing on
Windows because self.build_dir is set to a hard-coded absolute path
that is only valid on the non-Windows runners:

```
Relocating from /builds/gstreamer/cerbero/cerbero-build to C:/cerbero/cerbero-build
sed: no input files
WARNING: Could not retrieve dependencies for commit 7a55b396c380c47733b7a38ddae976656cde37f7:
Running 'sh -c "grep -lnrIU /builds/gstreamer/cerbero/cerbero-build | xargs sed "s#/builds/gstreamer/cerbero/cerbero-build#C:/cerbero/cerbero-build#g" -i"' returned 123
```

https://gitlab.freedesktop.org/gstreamer/gst-ci/-/jobs/4595909#L346

Part-of: <https://gitlab.freedesktop.org/gstreamer/cerbero/-/merge_requests/608>

nirbheek/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero