mkosi: Switch to fedora 40

Enable updates-testing and use the most recent mirror to make sure
we get util-linux 2.40.1 which contains a crucial fix to make sure
the serial terminal in virtual machines works properly.

.github/workflows/mkosi.yml

@@ -61,7 +61,7 @@ jobs:
           - distro: ubuntu
             release: noble
           - distro: fedora
-            release: "39"
+            release: "40"
           - distro: fedora
             release: rawhide
           - distro: opensuse

mkosi.conf.d/10-fedora.conf

@@ -4,4 +4,4 @@
 Distribution=fedora
 
 [Distribution]
-@Release=39
+@Release=40

mkosi.conf.d/20-fedora-40.conf

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+[Match]
+Distribution=fedora
+Release=40
+
+[Distribution]
+Repositories=updates-testing
+Mirror=https://dl.fedoraproject.org/pub