CI recompiles Zebra from scratch for each CI test

[upbqdn]

CI compiles Zebra upon each PR commit when preparing the Docker image for tests here: https://github.com/ZcashFoundation/zebra/actions/runs/13837431819/job/38722773169?pr=9323#step:10:871. However, each test compiles Zebra from scratch again. For example, here: https://github.com/ZcashFoundation/zebra/actions/runs/13837431819/job/38722794636?pr=9323#step:14:146.

This makes CI slow and expensive since it compiles Zebra from scratch ~ 15 times each time a PR is updated with a commit. I haven't looked at all the tests, but in some, the compilation takes longer than the test itself. CI should cache the compilation results and recompile only what's necessary, not only between image preparation and tests but also between individual preparations. We have the latter implemented: https://github.com/zcashfoundation/zebra/blob/49741e8b476aff1d1dd2f6a2f57ec9d24404d345/docker/Dockerfile#L97-L126, but it doesn't work. CI should also cache downloaded crates instead of pulling them for each compilation.

[gustavovalverde]

Do we want to re-open #6169 ?