Don't read password for generate, help or self-update

Fixes #2951.

changelog/unreleased/issue-2951

@@ -0,0 +1,9 @@
+Bugfix: restic generate, help and self-update no longer check passwords
+
+The commands `restic cache`, `generate`, `help` and `self-update` don't need
+passwords, but they previously did run the RESTIC_PASSWORD_COMMAND (if set in
+the environment), prompting users to authenticate for no reason. They now skip
+running the password command.
+
+https://github.com/restic/restic/issues/2951
+https://github.com/restic/restic/pull/2987