The setuid helper of D-Bus is responsible for "service activation".
It looks for '.service' files in fixed locations, but the default locations
make no sense (see below), so replace them with /etc/dbus-1/system-services.

--- dbus-1.10.0/dbus/dbus-sysdeps-util-unix.c	2015-10-22 00:07:03.829251854 +0200
+++ dbus-1.10.0/dbus/dbus-sysdeps-util-unix.c	2015-10-22 00:07:14.893445175 +0200
@@ -1410,10 +1410,7 @@ _dbus_get_standard_system_servicedirs (D
    * be available.
    */
   static const char standard_search_path[] =
-    "/usr/local/share:"
-    "/usr/share:"
-    DBUS_DATADIR ":"
-    "/lib";
+    "/etc";
   DBusString servicedir_path;
 
   _dbus_string_init_const (&servicedir_path, standard_search_path);
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<tr><td><span title='2022-01-10 11:44:29 -0500'>2022-01-10</span></td><td><a href='/guix/commit/tests/build-utils.scm?id=7f8a896c5f3ecde60a19f244d5a407ec1033a08d'>utils: Fix wrap-script argument handling.</a><span class='msg-avail'>...</span></td><td>Brendan Tildesley</td></tr>
<tr><td><span title='2021-06-04 22:34:26 +0200'>2021-06-04</span></td><td><a href='/guix/commit/tests/build-utils.scm?id=5378edeab4e90f9dec6ae9bc5706a4ac790294b7'>utils: Define ‘search-input-file’ procedure.</a><span class='msg-avail'>...</span></td><td>Maxime Devos</td></tr>
<tr><td><span title='2021-01-08 14:10:33 -0500'>2021-01-08</span></td><td><a href='/guix/commit/tests/build-utils.scm?id=485ac28235bf8775914b905060266b09629c1bac'>utils: Allow text substitution even in the presence of NUL characters.</a><span class='msg-avail'>...</span></td><td>Mark H Weaver</td></tr>
<tr><td><span title='2020-11-16 11:21:42 +0100'>2020-11-16</span></td><td><a href='/guix/commit/tests/build-utils.scm?id=977eb5d023cfdf8e336f1896480eea9cef5c04e9'>Properly deal with build directories containing '~'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-11-26 00:16:35 +0100'>2019-11-26</span></td><td><a href='/guix/commit/tests/build-utils.scm?id=f5698dfb87df1132ca5c73a8ed7f75f63c07669f'>tests: Avoid unnecessary use of 'mock'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-11-26 00:16:35 +0100'>2019-11-26</span></td><td><a href='/guix/commit/tests/build-utils.scm?id=1d64afbdc0644a2be8bd2ad157085731e212ab74'>tests: Avoid (catch 'srfi-34 …) form.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-06-17 16:13:36 +0200'>2019-06-17</span></td><td><a href='/guix/commit/tests/build-utils.scm?id=45d46223f92b0933aaf9b1392a21d09eaa1e2881'>utils: Add 'invoke/quiet'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-03-23 23:16:55 +0100'>2019-03-23</span></td><td><a href='/guix/commit/tests/build-utils.scm?id=8c14f7f8a7ab0722bf4c9f92fd28ae85514d564f'>Merge branch 'staging' into core-updates</a></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-03-16 18:15:13 +0100'>2019-03-16</span></td><td><a href='/guix/commit/tests/build-utils.scm?id=f0cc5e7e1e4c03af29c5d4855dc5962502c49147'>booloader: Add 'invoke/quiet'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-03-16 18:15:09 +0100'>2019-03-16</span></td><td><a href='/guix/commit/tests/build-utils.scm?id=22f95e028f038cee342f455dfc55bd32b804907c'>tests: Add 'with-environment-variable'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-02-08 10:26:53 +0100'>2019-02-08</span></td><td><a href='/guix/commit/tests/build-utils.scm?id=0fb9a8df429a7b9f40610ff15baaff0d8e31e8cf'>guix: Add wrap-script.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>