Added conditional dependancy downloading in CI

[dorryspears]

Progresses #1310

This will skip downloading the cargo dependencies if the cache is present. The cache gets refreshed based on the hash of the Cargo.toml file, so when that file doesn't change, this could save 2 - 3 minutes per run.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.34%. Comparing base (8151bdb) to head (15c00d5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1471   +/-   ##
=======================================
  Coverage   87.34%   87.34%           
=======================================
  Files          15       15           
  Lines        5303     5303           
=======================================
  Hits         4632     4632           
  Misses        671      671           

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[dorryspears]

@joshlf If some of these commands need to be run regardless of the cache filling, I can split them up.

I have a feeling we still may want to run

• cargo check --workspace --tests
• cargo kani setup

zerocopy/.github/workflows/ci.yml

Lines 507 to 512 in 8151bdb

	cargo check --workspace --tests &> /dev/null &
	cargo metadata &> /dev/null &
	cargo install cargo-readme --version 3.2.0 &> /dev/null &
	cargo install --locked kani-verifier &> /dev/null &
	cargo install cargo-nextest &> /dev/null &
	cargo kani setup &> /dev/null &