nixos/test-driver: Specify /bin/sh shell when running a bourne shell script as the user

The test harness provides the commands it wishes to run in Bourne
syntax.  This fails if the user uses a different shell.  For example,
with fish:

  machine.wait_for_unit("graphical-session.target", "alice")

machine # fish: Unsupported use of '='. To run '-u`' with a modified environment, please use 'env XDG_RUNTIME_DIR=/run/user/`id -u`…'
machine # XDG_RUNTIME_DIR=/run/user/`id -u` systemctl --user --no-pager show "graphical-session.target"
machine # ^
machine # [   16.329957] su[1077]: pam_unix(su:session): session closed for user alice
error: retrieving systemctl info for unit "graphical-session.target" under user "alice" failed with exit code 127

nixos/lib/test-driver/test-driver.py

@@ -369,7 +369,7 @@ class Machine:
             q = q.replace("'", "\\'")
             return self.execute(
                 (
-                    "su -l {} -c "
+                    "su -l {} --shell /bin/sh -c "
                     "$'XDG_RUNTIME_DIR=/run/user/`id -u` "
                     "systemctl --user {}'"
                 ).format(user, q)