One-shot Shepherd services required to correctly setup Podman in the
Guix System, are failing to find executables upon boot. This patch
changes the executable references to absolute paths to avoid PATH
lookup.
* gnu/services/containers.scm (cgroups-fs-owner-entrypoint): Hardcode
bash path;
(rootless-podman-cgroups-fs-owner-service): Ditto;
(rootless-podman-fs-entrypoint): Hardcode mount path.
Change-Id: Id6a27cadf51326ce57af93f57809b77e28dbeaef
Signed-off-by: Ludovic Courtès <ludo@gnu.org>