repart: when copying files into vfat or similar, do not set ownership

repart: when copying files into vfat or similar, do not set ownership

$ mkdir /var/tmp/files
$ touch /var/tmp/files/a
$ mkdir /var/tmp/conf
$ cat >>/var/tmp/conf/esp.conf
[Partition]
Type=esp
Format=vfat
CopyFiles=/var/tmp/files:/
$ truncate /var/tmp/disk -s 300M
$ sudo systemd-repart --dry-run=no --empty=require --definitions=/var/tmp/conf /var/tmp/disk
...
Populating vfat filesystem.
Failed to copy '...' to '/run/systemd/mount-root/': Operation not permitted
(sd-copy) failed with exit status 1.

The issue is that if there's a file owned by non-root and we try to copy
it into a newly-created DOS partition, fchown fails:
  fchown(11</run/systemd/mount-root/...>, 1000, 1000) = -1 EPERM (Operation not permitted)
We want to ignore file ownership in such cases, so pass our own UID/GID
to copy_tree_at(), which turns the fchown into a noop and let's the
operation pass through.

Fixes #38863.