lib/sysroot: Use direct g_mkdtemp() for overlay tmpdir

lib/sysroot: Use direct g_mkdtemp() for overlay tmpdir

The new libglnx `glnx_mkdtempat()` uses autocleanups, which
is inconvenient for this use case where we *don't* want autocleanups.
Since we don't need it to be fd-relative, just directly invoke
`g_mkdtemp_full()` which is fine for this use case.

Prep for updating libglnx.

Closes: #1161
Approved by: jlebon