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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<div class='path'>path: <a href='/guix/log/?id=d7463e012555047615193afcfd8294381f050825'>root</a>/<a href='/guix/log/tests?id=d7463e012555047615193afcfd8294381f050825'>tests</a>/<a href='/guix/log/tests/guix-build-branch.sh?id=d7463e012555047615193afcfd8294381f050825'>guix-build-branch.sh</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/tests/guix-build-branch.sh?id=d7463e012555047615193afcfd8294381f050825&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2018-11-30 17:03:04 +0100'>2018-11-30</span></td><td><a href='/guix/commit/tests/guix-build-branch.sh?id=a3d77c51bc8f641c12989e18dc5e03add776f87e'>git: Nicely report '--with-commit' errors.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/git.scm (latest-repository-commit*): Rewrite to catch
'git-error'.
* po/guix/POTFILES.in: Add guix/git.scm.
* tests/guix-build-branch.sh: Test --with-commit errors.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-30 17:03:04 +0100'>2018-11-30</span></td><td><a href='/guix/commit/tests/guix-build-branch.sh?id=b18f7234aac9eb42097c1b4cda7efe0be5aab132'>guix build: Add '--with-commit'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/git.scm (&lt;git-checkout&gt;)[commit]: New field.
(git-checkout-compiler): Honor it.
* guix/scripts/build.scm (evaluate-git-replacement-specs): Add 'proc'
parameter and honor it.
(transform-package-source-branch)[replace]: New procedure.
Adjust 'evaluate-git-replacement-specs' accordingly.
(transform-package-source-commit): New procedure.
(%transformations, %transformation-options)
(show-transformation-options-help): Add 'with-commit'.
* tests/guix-build-branch.sh: Add test.
* doc/guix.texi (Package Transformation Options): Document it.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-30 17:03:04 +0100'>2018-11-30</span></td><td><a href='/guix/commit/tests/guix-build-branch.sh?id=96915a448cfe8383a1c47f4b9a1cc810e5161fd0'>guix build: Add '--with-branch' transformation option.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/scripts/build.scm (evaluate-git-replacement-specs)
(transform-package-source-branch): New procedures.
(%transformations, %transformation-options): Add 'with-branch'.
(show-transformation-options-help): Likewise.
* tests/guix-build-branch.sh: New file.
* Makefile.am (SH_TESTS): Add it.
* doc/guix.texi (Package Transformation Options): Document it.
</span></span></td><td>Ludovic Courtès</td></tr>