deploy: set umask followup

For scap deploy, set the umask in _load_config() which runs
earlier that main(). The call to self.context.setup() in
_load_config() may result in files/directories being created, so the
umask needs to be set before that.

Also: Remove the git.is_dir() check from main(). The real check
happens earlier in _load_config() as of
f82a6acb. It's too late to check in
main().

Bug: T364309