CI: fix Tezt static tests

Context

The Tezt static tests didn't actually test static executables, because the artifacts were in the wrong place. Instead, those jobs rebuilt everything, but without the static flag, and used the resulting non-static executables.

I have thus:

• modified the job to use the right executables, by moving the artefacts fetched from the static job to the root: mv tezos-binaries/x86_64/tezos-* .
• removed the automatic "rebuild everything if executables are not here" mechanism, because it will never be right for static executables (we could modify it to have it rebuild static executables instead, but I don't think it's necessary)

After those changes, the jobs failed because coverage reports were not available. I'm not sure but maybe the coverage reports require some compilation flags that are not present when compiling statically? In any case, those reports are not very useful because the tests that we run on static executables are a subset of the tests that we run on non-static executables. Removing coverage reports for static executables should thus not reduce the apparent coverage report. So I also removed coverage reports for those jobs.

This should also have the side-effect of making the jobs run faster since they don't have to recompile everything (although this should be partially offset by the fact that static executables are slower to start).

Manually testing the MR

Check the CI, in particular:

• look at what the static Tezt jobs are doing;
• and check that coverage reports are not broken for other jobs.

Checklist

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• Select as Assignee the next person who should take action on that MR