github actions: remove rust-cache

[rillian]

Now that we're trying to review changes to github action scripts, it may not be worthwhile to use the cache. Experiment with removing it to see how that affects ci performance.

[rillian]

There's a lot of variance in the runtimes for test and style jobs, so while the runtimes with this change are longer than the immediately previous ones, they're not particularly longer than the recent history, even excepting the previous change to the cache code. No difference with the security or clippy runs.

Given that and (new?) telemetry code added by #350, I prefer removing the cache in favour of tracking one less dependency.

[claucece]

Agreed. this makes sense.