mkosi CI: mask isc-dhcp-server units

The packages are installed to provide the dhcpd binary, used by
test/test-network/systemd-networkd-tests.py, but we don't need the units
to run, and in fact in some cases the image fails to boot because of
them:

Spawning container image on /home/runner/work/systemd/systemd/image.raw.
Press ^] three times within 1s to kill container.
● isc-dhcp-server.service  loaded failed failed ISC DHCP IPv4 server
● isc-dhcp-server6.service loaded failed failed ISC DHCP IPv6 server
Container image failed with error code 1.
Error: Process completed with exit code 1.

Mask the units with an --extra-tree.

.github/workflows/run_mkosi.sh

@@ -7,11 +7,20 @@ set -o pipefail
 
 EC=0
 TEMPFILE="$(mktemp)"
-trap "rm -f '$TEMPFILE'" EXIT
+TEMP_EXTRA_TREE="$(mktemp --directory)"
+trap "rm -rf '$TEMPFILE' '$TEMP_EXTRA_TREE'" EXIT
+
+# We need isc-dhcp-server to be installed for the networkd unit tests, but we don't want to
+# run it by default. mktemp creates the directory as 700, so change it, otherwise it will
+# affect the image's root folder permissions.
+chmod 755 "$TEMP_EXTRA_TREE"
+mkdir -p "$TEMP_EXTRA_TREE/etc/systemd/system/"
+ln -s /dev/null "$TEMP_EXTRA_TREE/etc/systemd/system/isc-dhcp-server.service"
+ln -s /dev/null "$TEMP_EXTRA_TREE/etc/systemd/system/isc-dhcp-server6.service"
 
 for ((i = 0; i < 5; i++)); do
     EC=0
-    (sudo python3 -m mkosi "$@") |& tee "$TEMPFILE" || EC=$?
+    (sudo python3 -m mkosi --extra-tree="$TEMP_EXTRA_TREE" "$@") |& tee "$TEMPFILE" || EC=$?
     if [[ $EC -eq 0 ]]; then
         # The command passed — let's return immediately
         break