ci/flake: E2E `Theme switcher` disabled

Created by: valerybugakov

Test

https://github.com/sourcegraph/sourcegraph/blob/18c0a74db16d3dba470d46950d4b09b02ba99e77/client/web/src/end-to-end/frontend-platform/theme-switcher.test.ts#L36

Example failure

https://buildkite.com/sourcegraph/sourcegraph/builds/158869#0181d685-2ddd-48e9-84b0-2a3e16ed7f1f

Disabling PR

https://github.com/sourcegraph/sourcegraph/pull/38369

Additional details

No response