Cache dependencies in CI to speed up build times.

[adamreichold]

I am not sure how effective this is due mixing stable and nightly within a workflow and whether it was tried before, but I have had good experiences using the rust-cache action myself.

[adamreichold]

An unscientific experiment pushing the same commit with a different timestamp and hence hash leads to the following numbers:

job	cold	warm
coverage	22.5m	12m
test	13.5m	14m
test (cargo-nextest)		11m

So a significant win for the coverage job, but no improvement for the test job.

EDIT: cargo-nextest seems to help a wee bit with the test job but I am not sure whether it is worth the additional complexity (especially since the test runner does not support doctests yet).

[codecov-commenter]

Codecov Report

Merging #1469 (be2f5c1) into main (8b3a6f6) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1469      +/-   ##
==========================================
- Coverage   94.09%   94.09%   -0.01%     
==========================================
  Files         238      238              
  Lines       44552    44552              
==========================================
- Hits        41923    41921       -2     
- Misses       2629     2631       +2     

Impacted Files	Coverage Δ
bitpacker/src/bitpacker.rs	96.03% <0.00%> (-1.00%)	⬇️
src/fastfield/reader.rs	87.73% <0.00%> (-0.62%)	⬇️
src/store/index/mod.rs	97.83% <0.00%> (-0.55%)	⬇️
src/postings/stacker/expull.rs	99.05% <0.00%> (+0.47%)	⬆️

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[fulmicoton]

@adamreichold Great win! Thank you!