cpc/hooks: fix hang when running adt locally

While attempting to run autopkgtest locally, the test stops at the
following command:
ssh-keygen -t ed25519 -C ubuntu_vagrant_insecure_key -b 4096 -f
/tmp/tmp.VuAfnsBv1G/vagrant_insecure_key

This is found in live-build/ubuntu-cpc/hooks.d/base/vagrant.binary

It appears to be waiting for a passphrase, as running that outside of
adt gives a more helpful "Enter passphrase" prompt.

Explicitly set the passphrase to empty with the `-N` argument.

live-build/ubuntu-cpc/hooks.d/base/vagrant.binary

@@ -66,7 +66,8 @@ trap cleanup_vagrant EXIT
 ### https://github.com/hashicorp/vagrant/tree/main/keys
 ##########################
 
-ssh-keygen -t ed25519 -C "ubuntu_vagrant_insecure_key" -b 4096 -f ${box_d}/vagrant_insecure_key
+ssh-keygen -t ed25519 -C "ubuntu_vagrant_insecure_key" -b 4096 \
+	-f ${box_d}/vagrant_insecure_key -N ""
 
 pub_key=$(cat ${box_d}/vagrant_insecure_key.pub)