boot: Add ostree-finalize-staged.path

Rather than manually starting the `ostree-finalize-staged.service` unit,
we can leverage systemd's path units for this. It fits quite nicely too,
given that we already have a path we drop iif we have a staged
deployment.

To give some time for the preset to make it to systems, we don't yet
drop the explicit call to `systemctl start`. Though we do make it
conditional based on a DEBUG env var so that we can actually test it in
CI for now. Once we're sure this has propagated, we can drop the
`systemctl start` path and the env var together.

Closes: #1740
Approved by: cgwalters

src/libostree/ostree-sysroot.c

@@ -189,6 +189,7 @@ ostree_sysroot_init (OstreeSysroot *self)
     { "mutable-deployments", OSTREE_SYSROOT_DEBUG_MUTABLE_DEPLOYMENTS },
     { "test-fifreeze", OSTREE_SYSROOT_DEBUG_TEST_FIFREEZE },
     { "no-xattrs", OSTREE_SYSROOT_DEBUG_NO_XATTRS },
+    { "test-staged-path", OSTREE_SYSROOT_DEBUG_TEST_STAGED_PATH },
   };
 
   self->debug_flags = g_parse_debug_string (g_getenv ("OSTREE_SYSROOT_DEBUG"),