mkosi: Use build/ as extra search path by default

Building systemd with mkosi generally requires a very recent version
of systemd which might not be installed on the host. Let's configure
mkosi to look for extra executables in the build/ directory by default
so that we prefer systemd executables from the build directory over those
on the host as those on the host are likely to be too old.

mkosi.conf.d/20-extra-search-path.conf

@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+[Match]
+PathExists=build/
+
+[Build]
+ExtraSearchPaths=build/