Rust/Cargo CI perf improvements

[jsirois]

A few ideas stumbled upon while trying to speed up CI burns on #6104:

• Cache src/rust/engine/target in .travis.yml
  The official rust language support in Travis caches ~/.cargo and target/, so this is kosher from their point of view and blessed by Chrichton. I tried this already and the upshot is all 3rdparty crate compilations are cached when they have not changed versions - easy win.
• Throttle cargo build as we do for zinc in pants.travis-ci.ini
  This can be done via CARGO_BUILD_JOBS=N in .travis.yml and might give us better throughput with less thrash. Measurements needed.
• Throttle cargo test similar to above.
  Not sure how to best do this. There is rumor RUST_TEST_THREADS=N works, though it looks discouraged. I've had great luck with cargo make ... which allows setting up arbitrary commands in a Makefile.toml on a side project. Might be a way to go for these bottom two bullets. Likely very little bang for the buck here though, tests are still pretty fast.

[illicitonion]

We already throttle cargo test for OSX:

pants/build-support/bin/ci.sh

Lines 220 to 224 in cafd732

	if [[ "$(uname)" == "Darwin" ]]; then
	# The osx travis environment has a low file descriptors ulimit, so we avoid running too many
	# tests in parallel.
	test_threads_flag="--test-threads=1"
	fi

easy to extend to Linux if needed (but realistically, that shard runs pretty fast)

[jsirois]

Hitting the 1st bullet in #6208

[jsirois]

Alright, #6309 cuts ~4 minutes per shard. I'm going to stop progress here and close.