Remove built dependency

[Jake-Shadle]

This was pulling in git2 and writing a lot of data that was never used
in the crate. This just changes build.rs to manually read the HEAD
commit information and set an env var.
AFAICT this is actually superior in every way to built since we no
longer have the extra dependencies, and we actually get up to date
information when the commit changes, which AFAICT was not the case with
built since it was not registering file changes.

[XAMPPRocky]

These tests weren't flaky until today 😄

[markmandel]

These tests weren't flaky until today 😄

Yeah! It's being really grumpy! I wonder what's up with that.

If you see a flake, please chuck a comment in the PR of a c/p paste of the flake in the PR, and I'll do the same - that makes it easier to search for common flakes through GitHub search and eventually resolve (also we delete old build failures, so it's harder to see the history).

[XAMPPRocky]

If you see a flake, please chuck a comment in the PR of a c/p paste of the flake in the PR, and I'll do the same - that makes it easier to search for common flakes through GitHub search and eventually resolve (also we delete old build failures, so it's harder to see the history).

It wasn't even the same test though which is odd, it was different tests.

[quilkin-bot]

Build Succeeded 🥳

Build Id: 2dbf39bb-babb-4613-8999-3c13c40a464b

The following development images have been built, and will exist for the next 30 days:

• us-docker.pkg.dev/quilkin/ci/quilkin:0.8.0-dev-05c2fe6

To build this version:

git fetch git@github.com:googleforgames/quilkin.git pull/847/head:pr_847 && git checkout pr_847
cargo build

[markmandel]

It wasn't even the same test though which is odd, it was different tests.

Could be Cloud Build is having a grumpy day.