Use rust-cache GitHub action in Wasmi's CI

[Robbepop]

The problem with the current cargo-cache setup is that it still re-compiles all crates each run and only avoids re-downloading all the dependencies.
The rust-cache GitHub action promises to also avoid re-compiling all dependencies.

This PR also fixes incorrect target/corpus directories for the fuzzing CI jobs.

Link: https://github.com/Swatinem/rust-cache

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.48%. Comparing base (ece0757) to head (538f145).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1098   +/-   ##
=======================================
  Coverage   80.48%   80.48%           
=======================================
  Files         270      270           
  Lines       25079    25079           
=======================================
  Hits        20184    20184           
  Misses       4895     4895           

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[Robbepop]

Results: we dropped CI times from sometimes well over 15 min to roughly 4-5 minutes.
Especially the test jobs improved substantially and are now at around 1-2 minutes due to non-optimizing test builds.
Also fuzzing jobs now finally properly cache their build and corpus artifacts.