Multiple simultaneous requests result in HTTP 304

Certain test suites in the Playwright E2E project intermittently encounter HTTP 304 status when first attempting to load a calypso URL.

The failure is presumably occurring [here](https://github.com/Automattic/wp-calypso/blob/trunk/packages/calypso-e2e/src/lib/flows/login-flow.ts#L65) when the `/log-in` endpoint is being hit by multiple containers at the same time. Retrying (automatically via `magellan`) usually results in the next iteration being able to access the endpoint as normal.

Slack thread: p1624428683011700-slack-C01BVMH5C20

Start at packages/calypso-e2e/src/lib/flows/login-flow.ts around line 65 and inspect how concurrent containers request the Calypso login URL. Run the affected Playwright E2E suites with multiple containers, compare the initial request with Magellan's retry, and confirm that simultaneous first loads no longer intermittently return HTTP 304.

Written by the indexing model from the issue text.