[Merge] ~dbungert/livecd-rootfs:adt-hang-vagrant into livecd-rootfs:ubuntu/master

[Dan Bungert]

The proposal to merge ~dbungert/livecd-rootfs:adt-hang-vagrant into livecd-rootfs:ubuntu/master has been updated.

Commit message changed to:

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.

An example invocation of autopkgtest, which produces a hang without this fix or successful build with this fix:

autopkgtest --no-built-binaries --test-name=default-bootstraps --shell --apt-upgrade livecd-rootfs -- qemu autopkgtest-mantic-amd64.img --ram-size=8192 --cpus=16

the .img file was created with autopkgtest-buildvm-ubuntu-cloud

For more details, see:
https://code.launchpad.net/~dbungert/livecd-rootfs/+git/livecd-rootfs/+merge/449308
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