Mount over /etc/passwd when building an image and running build script

Mount over /etc/passwd when building an image and running build script

Various tools parse /etc/passwd, so let's make sure it contains the
expected lines when building an image. Also, when running the build
script, mount over /etc/passwd in the image as various build tools
will also query the current user in /etc/passwd (e.g. the kernel's
build script runs "whoami"). Also add a root entry to the /etc/passwd
we generate so both the user running mkosi and root are covered.