Caching breaks showcore workaround

Problem/Motivation

The 'showcore' query string workaround for ensuring the core login form shows, which was added in #2478487: Modify the core user login form to show external providers, is not compatible with caching the form.

Steps to reproduce

1. Enable caching and the setting to replace the core login form.
2. Visit /user/login . The core form does not show. (good)
3. Load /user/login?showcore ... the form still does not show. (bad)
4. Clear caches
5. Immediately load /user/login?showcore . The core form shows (good)
6. Load /user/login ... the core form still shows (bad!)

Proposed resolution

Add a cache context to the form

Remaining tasks

Verify the patch.

User interface changes

The form shows/hides correctly.

API changes

None.

Data model changes

None.