.gitlab-ci.yml:  Disable cargo publish for now, since we're relying on a fork of one of our dependencies

.gitlab-ci.yml:  Run `cargo check` instead of `cargo build` to avoid some of the build time; run `cargo build` after everything else runs

.gitlab-ci.yml:  Remove separate `deploy-dryrun` step and just do `cargo publish --dry-run` in main cargo build/clippy/test step

.gitlab-ci.yml:  Use tools/maybe-download-sccache instead of hardcoding basically the same script directly into here

.gitlab-ci.yml:  It costs more time to save the builder images (even ignoring loading them) than it does to just build them from scratch, so stop trying to cache those; just save the release image so that `docker push` step can use it

.gitlab-ci.yml:  Since Gitlab CI cache isn't currently working, nuke the docker save/load steps; they're taking forever (6+ minutes) and providing no value

This reverts commit f3298765.

.gitlab-ci.yml:  Fix sccache (hopefully) in `cargo build` step, add `cargo clippy` to it, and move `cargo test` into it