mkosi: drop os-release symlink for minimal-base image

[  385s] ERROR: link target doesn't exist (neither in build root nor in installed system):
[  385s]   /usr/lib/systemd/tests/mkosi/mkosi.images/minimal-base/mkosi.extra/etc/os-release -> ../usr/lib/os-release

It shouldn't be even needed, everything should look in /usr/lib/os-release too

Follow-up for a0ce5e642f

(cherry picked from commit 5e5f792193)

mkosi/mkosi.images/minimal-base/mkosi.extra/etc/os-release

@@ -1 +0,0 @@
-../usr/lib/os-release

mkosi/mkosi.images/minimal-base/mkosi.postinst

@@ -9,3 +9,8 @@ echo "Hello from dummy init, beautiful day, innit?"
 ip link
 EOF
 chmod +x "$BUILDROOT/sbin/init"
+
+# Arch relies on tmpfiles.d/etc.conf but this image isn't booted
+if [ ! -e "$BUILDROOT/etc/os-release" ]; then
+    ln -s ../usr/lib/os-release "$BUILDROOT/etc/os-release"
+fi