chore(ci): add cargo build caching to ci pipeline

[LNSD]

Running the CI pipeline can take more than 10 minutes. And in the case the integration test fail (due to external reasons), one would have to rerun the CI job, consuming another 10 minutes.

• This PR adds a "cargo build cache" step that caches the build artifacts to optimize significantly for the CI execution time. Reducing the execution time from minutes to a few seconds.
• The unit and integration test steps have also been split to make spotting the failing test case easier.
• Limited the CI jobs concurrency to only one per PR. When a new push happens, the previous build is canceled.
• CI builds will only happen when "a PR is opened, or a push happens on a PR" and when a PR is merged into the main branch.

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As Seeing is believing 👀:

First run (no cache):

Reusing an existing cache:

[Theodus]

Awesome! I was hoping there was a way to speed this up.