Add Rust Cache Action to Github Actions

[smklein]

This PR integrates the Rust Cache Action to cache third party dependencies. It exclusively applies them to Github workflows, and does not yet optimize Buildomat. The cache step is skipped on main so it will always build from scratch there.

Before Cache:

After Cache:

[smklein]

Note: this does not yet add a cache for the buildomat action, but it at least makes the Github actions much faster when third-party dependencies are unchanged.

[david-crespo]

This means it worked, right? Holy crap.

[david-crespo]

incredible!

[smklein]

This means it worked, right? Holy crap.

I believe this particular image is referencing the cache for Clickhouse/Cockroach being downloaded:

omicron/.github/workflows/rust.yml

Lines 97 to 116 in ab8872a

	- name: Configure GitHub cache for CockroachDB binaries
	id: cache-cockroachdb
	# actions/cache@v2.1.4
	uses: actions/cache@26968a09c0ea4f3e233fdddbafd1166051a095f6
	with:
	key: ${{ runner.os }}-cockroach-binary-${{ hashFiles('tools/cockroachdb_checksums') }}
	path: "cockroachdb"
	- name: Configure GitHub cache for ClickHouse binaries
	id: cache-clickhouse
	# actions/cache@v2.1.4
	uses: actions/cache@26968a09c0ea4f3e233fdddbafd1166051a095f6
	with:
	key: ${{ runner.os }}-clickhouse-binary-${{ hashFiles('tools/clickhouse_checksums') }}
	path: "clickhouse"
	- name: Download ClickHouse
	if: steps.cache-clickhouse.outputs.cache-hit != 'true'
	run: ./tools/ci_download_clickhouse
	- name: Download CockroachDB binary
	if: steps.cache-cockroachdb.outputs.cache-hit != 'true'
	run: bash ./tools/ci_download_cockroachdb

This is the part of the action where we check for a cache:

[david-crespo]

I meant the build taking 5 minutes!

[smklein]

I meant the build taking 5 minutes!

oh yeah I hope so lol

[david-crespo]

As discussed in chat, is there a way to make the use of the cache conditional so we can skip it on main? We want to be running clean builds at least some of the time, and main is a good spot for that because we're never waiting for CI to pass there. We would also have to turn off whatever it is that skips builds on main when it thinks the PR already handled it.

[david-crespo]

Branch skipping works. Tested by telling it to skip this branch instead of main: