mkosi: Conditionally use tools tree

If the systemd version on the host is too old and there's no local
build directory, use the default tools tree which will build an
image containing all the tooling required to build systemd and use
that to build the other presets.

.github/workflows/mkosi.yml

@@ -80,9 +80,6 @@ jobs:
 
     - name: Configure
       run: |
-        # We configure ExtraSearchPaths=build/ so make sure build/ exists.
-        mkdir build/
-
         tee mkosi.conf <<- EOF
         [Distribution]
         Distribution=${{ matrix.distro }}

mkosi.conf.d/10-extra-search-paths.conf

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

mkosi.conf.d/10-systemd.conf

@@ -17,7 +17,6 @@ Environment=ASAN_OPTIONS=verify_asan_link_order=false
 [Host]
 @Incremental=yes
 @QemuMem=2G
-ExtraSearchPaths=build/
 # Make sure we don't trigger systemd-firstboot prompting for the root password.
 Credentials=passwd.plaintext-password.root=
 KernelCommandLineExtra=systemd.crash_shell

mkosi.conf.d/10-tools.conf

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+[Match]
+PathExists=!build/
+SystemdVersion=<254
+
+[Host]
+ToolsTree=default