Fetch the latest commit from the crates index before syncing

Created by: olafurpg

Previously, the crates syncer job could index a stale commit from rust-lang/crates.io-index because we didn't trigger a "git fetch" on the repo before calling "git archive". This commit, adds a "git fetch" step so that we always index the latest commit from the index.

Fixes https://github.com/sourcegraph/sourcegraph/issues/37690

Test plan

Deploy the change and manually observer that we're picking up new releases. Once we complete https://github.com/sourcegraph/sourcegraph/issues/38656 we'll have better infrastructure to test changes like this.