Re-rache repository locations on WorkspaceProcessContext everytime a request is made
Previously, the repository locations from the workspace were cached on the WorkspaceProcessContext when initialized. This is because the process context assumes that the workspace only changes when reload_workspace is called on the ProcessContext. With the new async workspaces, this is not true anymore.
This diff changes the process context to not cache the locations anymore and directly access the underlying workspace instead
Test Plan: Existing tests
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D7682