unbreak dagit read-only mode

Summary: While testing some read-only related changes it became clear that a bad refactor caused the read_only param to the dagit CLI to get thrown out :( This diff threads it through correctly.

Test Plan: BK, load dagit with --read-only and verify you can't launch pipelines

Reviewers: alangenfeld, johann, jordansanders, rexledesma

Reviewed By: rexledesma

Differential Revision: https://dagster.phacility.com/D8811