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-private.h

@@ -36,6 +36,9 @@ typedef enum {
   OSTREE_SYSROOT_DEBUG_NO_XATTRS = 1 << 1,
   /* https://github.com/ostreedev/ostree/pull/1049 */
   OSTREE_SYSROOT_DEBUG_TEST_FIFREEZE = 1 << 2,
+  /* This is a temporary flag until we fully drop the explicit `systemctl start
+   * ostree-finalize-staged.service` so that tests can exercise the new path unit. */
+  OSTREE_SYSROOT_DEBUG_TEST_STAGED_PATH = 1 << 3,
 } OstreeSysrootDebugFlags;
 
 /**