Short circuit discovery for XDG_RUNTIME_DIR paths

If the desired executable is mounted by the document portal,
then we can short-circuit host binary discovery and pass the
path directly to flatpak-spawn, which should do the right
thing automatically.

Change-Id: Iff0fffc42bc1dc3fa3e01a1c0974092e413e67c1
Signed-off-by: Mat Booth <mat.booth@redhat.com>
Reviewed-on: https://git.eclipse.org/r/c/linuxtools/org.eclipse.linuxtools.eclipse-build/+/177345