Use tarpaulin instead of grcov

[luizirber]

Rust coverage reports are very finicky, trying out https://github.com/actions-rs/tarpaulin/ instead

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #862 into master will decrease coverage by 0.6%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #862      +/-   ##
==========================================
- Coverage   78.89%   78.29%   -0.61%     
==========================================
  Files          82       94      +12     
  Lines        6993     7288     +295     
  Branches      479        0     -479     
==========================================
+ Hits         5517     5706     +189     
- Misses       1174     1582     +408     
+ Partials      302        0     -302

Flag	Coverage Δ
#pytests	?
#rusttests	50.66% <100%> (+3.36%)	⬆️

Impacted Files	Coverage Δ
src/core/tests/signature.rs	100% <100%> (ø)
src/core/src/index/linear.rs	7.57% <0%> (-84.1%)	⬇️
sourmash/signature.py	87.19% <0%> (-0.49%)	⬇️
sourmash/sbt_storage.py	85.39% <0%> (-0.33%)	⬇️
sourmash/command_compute.py	96.96% <0%> (-0.24%)	⬇️
sourmash/sbtmh.py	84.28% <0%> (-0.23%)	⬇️
sourmash/_minhash.py	97.08% <0%> (-0.13%)	⬇️
sourmash/sbt.py	86.81% <0%> (-0.08%)	⬇️
sourmash/index.py	98.38% <0%> (-0.08%)	⬇️
sourmash/lca/lca_utils.py	96.6% <0%> (-0.06%)	⬇️
... and 26 more

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[luizirber]

Ready for review and merge @ctb