Fix non-thread-safe calls to Workspace.repository_locations and WorkspaceProcessContext.repository_locations

Summary: These are the last two lingering non-thread-safe methods on Workspace (I think). Using these methods runs the risk that a background thread will be messing with the list of repository_locations under the hood. Instead, require callers (almost none exist outside of tests - workspace telemetry appears to be the only 'real' one) to create a request context .

Test Plan: BK

Reviewers: sashank, alangenfeld, prha

Reviewed By: alangenfeld

Differential Revision: https://dagster.phacility.com/D8049