flatpak-builder hardlinks between its state dir and the build dir, so they must
share a filesystem. With only the build dir on tmpfs it errored ('state dir not
on the same filesystem as the target dir'). Move .flatpak-builder to tmpfs as
well via FLATPAK_STATEDIR; its download cache no longer persists across --ram
runs, which is the accepted trade for the tmpfs speedup.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>