fix: better caching of Rust deps

[ntn-x2]

I chored up the CI file by making sure all jobs (minus the cargo-test job) use the same approach:

• Running the whole job INSIDE the Parity-provided Docker container
• Make sure the right cargo path as configured inside the container is used to cache cargo artifacts
• Use the docker run approach only for the cargo-test job, since the worker still runs out of memory and crashes

Main changes

• Use of container: component in all jobs but cargo-test
• Use of the /usr/local/cargo path for caching cargo artifacts. This is the path configured by the container itself. Using other paths won't have the expected effect.
• Addition of save-always: true for the caching of artifacts, since even if, for example a test fails, we still don't want to throw away the compilation work done to run that test
• The try-runtime job had a cache configured by it was not caching anything, since we were caching the wrong path. That's fixed by using the same approach all other jobs use.

[Ad96el]

Love it

[Ad96el]

It's out of scope for this PR, but do you think it would be worth changing the image to an environment variable? We have multiple workflow files, and using an environment variable would be less error-prone.

[ntn-x2]

This would need to be a repo-wide variable, which should be updated once we bump our dependencies to a new version, if we want to share it across workflows. I agree it's out of scope, but I cc @ggera to see if there's anything low effort we can do to minimize duplication.