CI improvements

[TriplEight]

• Rust nightly and beta tests are back and shiny, will run during the nightly build
• all test and build jobs changed to run with --verbose --color=always
• max cache size raised to 50 Gib
• log files are collected for linux jobs and uploaded to GitLab artifacts. But I didn't figure out how to have two artifacts: sections in one job, so for build stage logs will still be uploaded, but with the &collect_artifacts rule (on_success and 1 mos).

So I was testing pipelines with per-job sccache caching, thinking it was a last resort.
It turned out it was not necessary to resort to such extreme measures:
Random compilation failure bug seems to appear only when the max cache size allocated is exceeded, and fail happens when one job removes an arbitrary artifact and then the next job wants to remove the same one, but doesn't find it.

Another problem I thought was connected with caching, some tests were failing. At first I thought it's due to caching, but per-job isolation showed that it's not the caching problem.

As a bonus, we (with Niklas help) have found 2 flaky tests.

[parity-cla-bot]

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Parity Technologies CLA Bot

[gabreal]

looks also good to me. so there won't be tests for mac or win?

[TriplEight]

looks also good to me. so there won't be tests for mac or win?

@gabreal this is not decided yet, and will require changes on hosts.