repart: respect `SOURCE_DATE_EPOCH` on `mkdir_p_root`

repart: respect `SOURCE_DATE_EPOCH` on `mkdir_p_root`

This let's systemd-repart respect the `SOURCE_DATE_EPOCH` environment
variable when creating directories in the local tree through `CopyFiles`
or `MakeDirectories`.

To do this, we pass a timestamp `ts` to `mkdir_p_root`, which it will
use to fix up `mtime` and `atime` of the directory it creates as
well as the `mtime` of the directory it creates the other directory *in*,
as the `mtime` of the latter is modified when creating a directory in it.

For the same reason, it also needs to fixup the `mtime` of the upper
directory when copying a file into it through `CopyFiles`.

If `SOURCE_DATE_EPOCH`, times are left as is. (`UTIME_OMIT`)