mkosi: skip repo keys checks

F39 doesn't build anymore:

GPG key at https://fedoraproject.org/fedora.gpg (0x31645531) is already installed
Public key for filesystem-3.18-6.fc39.x86_64.rpm is not installed. Failing package is: filesystem-3.18-6.fc39.x86_64
 GPG Keys are configured as: https://fedoraproject.org/fedora.gpg
Public key for setup-2.14.4-1.fc39.noarch.rpm is not installed. Failing package is: setup-2.14.4-1.fc39.noarch
 GPG Keys are configured as: https://fedoraproject.org/fedora.gpg
The GPG keys listed for the "updates" repository are already installed but they are not correct for this package.
Check that the correct key URLs are configured for this repository.. Failing package is: fedora-gpg-keys-39-2.noarch
 GPG Keys are configured as: https://fedoraproject.org/fedora.gpg
Public key for fedora-release-39-36.noarch.rpm is not installed. Failing package is: fedora-release-39-36.noarch
 GPG Keys are configured as: https://fedoraproject.org/fedora.gpg
Public key for fedora-release-common-39-36.noarch.rpm is not installed. Failing package is: fedora-release-common-39-36.noarch
 GPG Keys are configured as: https://fedoraproject.org/fedora.gpg
Public key for fedora-release-identity-basic-39-36.noarch.rpm is not installed. Failing package is: fedora-release-identity-basic-39-36.noarch
 GPG Keys are configured as: https://fedoraproject.org/fedora.gpg
Public key for fedora-repos-39-2.noarch.rpm is not installed. Failing package is: fedora-repos-39-2.noarch
 GPG Keys are configured as: https://fedoraproject.org/fedora.gpg
Error: GPG check FAILED

These are throw-away CI images, so just skip the checks

Author: bluca

.github/workflows/mkosi.yml

@@ -138,7 +138,7 @@ jobs:
       run: mkosi summary
 
     - name: Build
-      run: sudo mkosi --debug
+      run: sudo mkosi --debug --repository-key-check=false
 
     - name: Boot systemd-nspawn
       run: test "$(sudo mkosi --debug boot 1>&2; echo $?)" -eq 123