Get wayland-session wrapper from environment.

diff --git a/daemon/gdm-session.c b/daemon/gdm-session.c
index 3b83fcd3..e60e6a57 100644
--- a/daemon/gdm-session.c
+++ b/daemon/gdm-session.c
@@ -2933,8 +2933,9 @@ gdm_session_start_session (GdmSession *self,
                                                            allow_remote_connections? "--allow-remote-connections " : "",
                                                            command);
                         } else {
-                                program = g_strdup_printf (LIBEXECDIR "/gdm-wayland-session %s\"%s\"",
+                                program = g_strdup_printf (LIBEXECDIR "/gdm-wayland-session %s\"%s %s\"",
                                                            register_session ? "--register-session " : "",
+                                                           g_getenv ("GDM_WAYLAND_SESSION"),
                                                            command);
                         }
                 } else if (run_xsession_script) {
@@ -2965,8 +2966,9 @@ gdm_session_start_session (GdmSession *self,
                                                            register_session ? "--register-session " : "",
                                                            self->selected_program);
                         } else {
-                                program = g_strdup_printf (LIBEXECDIR "/gdm-wayland-session %s\"dbus-run-session -- %s\"",
+                                program = g_strdup_printf (LIBEXECDIR "/gdm-wayland-session %s\"dbus-run-session -- %s %s\"",
                                                            register_session ? "--register-session " : "",
+                                                           g_getenv ("GDM_WAYLAND_SESSION"),
                                                            self->selected_program);
                         }
                 } else {
/table>
<div class='path'>path: <a href='/guix/log/?id=1261bc7ac855ac1742d515b15d52b8f4516c8a63'>root</a>/<a href='/guix/log/gnu?id=1261bc7ac855ac1742d515b15d52b8f4516c8a63'>gnu</a>/<a href='/guix/log/gnu/artwork.scm?id=1261bc7ac855ac1742d515b15d52b8f4516c8a63'>artwork.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/artwork.scm?id=1261bc7ac855ac1742d515b15d52b8f4516c8a63&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-06-10 23:11:07 +0200'>2019-06-10</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=226a3605d6cabeb71247a732a99d625c9cfafa74'>artwork: Update snapshot to 2f2fe74.</a><span class='msg-avail'>...</span></td><td>Diego Nicola Barbato</td></tr>
<tr><td><span title='2019-03-05 23:08:53 +0100'>2019-03-05</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=5387971a9ecf511e1dbbac992af3808437e58d56'>artwork: Update snapshot to b975164.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-02-10 23:25:01 +0100'>2019-02-10</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=5e6a5aeebd7b735e5d5f2eed496cc1c7e8a5b4c9'>artwork: Update snapshot to e951905.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-05 00:08:47 +0100'>2018-11-05</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=688d1a01e45eb1277a6337987bfd53f3fe3caaec'>artwork: Clone over HTTPS.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-06-07 15:58:16 -0400'>2017-06-07</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=94db92f78ec9c849f9c8da05d4d35356909f9fbe'>artwork: Use a descriptive name for the source directory.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>