fix(c10n): clean up temp file
fix(c10n): execute rather than source script-like user-data
fix(c10n): preserve initial user-data as c10n token
fix(c10n): do not exit on successful ssh-key injection

This exit should not have been added in the first place.
The user-data can still be used as a c10n token, even
if an SSH key was found after following the user-data
as a URL.
fix(c10n): use update-ssh-keys