meta-launcher: g_propagate_*_error takes ownership

This means we need to make sure we don't accidentally free the provided
source GError (which automatically happens with `g_autoptr`), so use
`g_steal_pointer()`.

This fixes an issue where, when launched in a bubblewrap environment
(such as the one provided by Buildstream), mutter would give the
following warning message:

```
mutter-WARNING **: 8:31:35:069: Can't initialize KMS backend: (null)
```

... which isn't that useful when trying to debug the actual issue.
2 jobs for master in 3 minutes and 37 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #213067
build-mutter

00:01:10

 
  Test
passed #213068
test-mutter

00:02:26