Allowed GHOST_URL to be customised in yarn dev:forward x Stripe (#25908)

no issue

- When running ActivityPub alongside Ghost, we use a public-facing url
for local development - so that it can interact with other actors from
the Fediverse. We typically use Tailscale funnel to proxy our localhost
to a public-facing url
- This change allows us to pass the public-facing url in the `yarn
dev:forward` dev setup with Stripe profile, so that Stripe webhooks are
forwarded to the correct url

How to use:
- Add `GHOST_URL=<YOUR_LOCAL_SITE_URL>` to the .env file at the root of
the monorepo
- If left empty, it will fallback to the default value of
`http://ghost-dev:2368`