systemd: sshd: Disable ssh-keygen

The default _sshd.service_ unit runs `ssh-keygen -A` in `ExecStartPre`,
to ensure host keys exist before starting the SSH daemon.
Unfortunately, this tool does _not_ respect the `HostKey` settings in
`sshd_config`, and unconditionally tries to create the key files in
`/etc/ssh`.  Since this directory is not writable, the command fails,
preventing the SSH daemon from starting.

build-rootfs.sh

@@ -90,7 +90,8 @@ qemu-${target%%-*} \
     -f /mnt/gentoo/etc/ld.so.conf
 
 if [ -d /mnt/gentoo/lib64/security ]; then
-    mv /mnt/gentoo/lib64/security/pam_*.so /mnt/gentoo/lib/security/
+    find /mnt/gentoo/lib64/security -name 'pam_*.so' \
+        -exec mv -t /mnt/gentoo/lib/security {} +
 fi
 
 rm -f /mnt/gentoo/lib/tmpfiles.d/provision.conf

overlay/usr/lib/systemd/sshd.service.d/no-keygen.conf

@@ -0,0 +1,2 @@
+[Service]
+ExecStartPre=