Update Homebrew formula to pull through Scarf endpoint

[arjundevarajan]

This PR updates the RisingWave Homebrew formula to fetch the RisingWave binary via a Scarf endpoint. This allows RisingWave maintainers to collect basic de-identified download and adoption metrics. It does not affect how previous versions of RisingWave are downloaded.

This change was suggested by the RisingWave product team in direct discussions. To test this, download RisingWave using the new endpoint (e.g. https://risingwave.gateway.scarf.sh/risingwave/archive/refs/tags/v1.8.1.tar.gz) and verify that the risingwavelabs/risingwave binary downloads without issue.

For future versions of RisingWave, the same formatting structure can be used without issue (with the version updated in the endpoint).

[alicelyy]

Verified docker pull successful:

➜  ~ docker pull risingwave.docker.scarf.sh/risingwavelabs/risingwave
Using default tag: latest
latest: Pulling from risingwavelabs/risingwave
ca6fd4ca81e0: Pull complete
59850338221a: Pull complete
e768fb86ff00: Pull complete
e3a663b20ee7: Pull complete
4eb3c6818fed: Pull complete
00009779bb37: Pull complete
db704c6aa20d: Pull complete
b6ebe865eadc: Pull complete
6df207a26781: Pull complete
85a70973c738: Pull complete
Digest: sha256:298e488129a9c9be2feb2af5ca3c23127f69114fb2c915cbfb432f8c671f02e1
Status: Downloaded newer image for risingwave.docker.scarf.sh/risingwavelabs/risingwave:latest
risingwave.docker.scarf.sh/risingwavelabs/risingwave:latest
➜  ~ docker pull risingwave.docker.scarf.sh/risingwavelabs/risingwave
➜  ~ docker pull ghcr.io/risingwavelabs/risingwave
Using default tag: latest
latest: Pulling from risingwavelabs/risingwave
Digest: sha256:298e488129a9c9be2feb2af5ca3c23127f69114fb2c915cbfb432f8c671f02e1
Status: Downloaded newer image for ghcr.io/risingwavelabs/risingwave:latest
ghcr.io/risingwavelabs/risingwave:latest