Make check{2,3} faster by enabling parallel rustc for dcou #34739
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Problem
dcou needs some bruteforce compilation for every crates to circumvent cargo's feature unification for its intended functioning. so, it's slow. and people (including me) are starting to be frustrated about it.
Summary of Changes
use recent rust's parallelization for front-end. this is especially effective, because the dcou check only doesn't care about generating binaries.
https://blog.rust-lang.org/2023/11/09/parallel-rustc.html
results (15%-25% faster):
before (taken from #34732):
buildkite/solana/checks2 Passed (7 minutes)
buildkite/solana/checks3 Passed (14 minutes, 50 seconds)
after:
buildkite/solana/checks2 Passed (5 minutes, 55 seconds)
buildkite/solana/checks3 Passed (10 minutes, 52 seconds)
related: #32169