Investigate sccache

[mjkoo]

Is your feature request related to a problem? Please describe. Relay build times in e2e are pretty long, see if we can set up https://github.com/mozilla/sccache for our CI runners to speed this up

Perhaps relevant to gitlab CI: https://github.com/mozilla/sccache/blob/master/docs/Jenkins.md

Rough plan from quick research:

• [x] Set up s3 bucket for artifacts
• [x] Set SCCACHE_BUCKET, AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY env vars for CI runs
• [x] Install sccache into docker image before cargo steps, fetch static bin from https://github.com/mozilla/sccache/releases probably
• [x] Set RUSTC_WRAPPER env var to point to sccache

[myhndl]

takes about 30 mins

[polyswarm-pm-admin]

Available as AWS_GITLAB_PROD_ACCESS_KEY_ID and AWS_GITLAB_PROD_SECRET_ACCESS_KEY.

[mjkoo]

Done in #61