Rust

Allow specifying a path to `xtask-downloader` via env var (#6214) #35464

Job	Run time
check-omicron-deployment (ubuntu-22.04)	7m 27s
check-features	10m 25s
build-docs	7m 14s
check-omicron-deployment (macos-14)	7m 16s
check-style	17s
clippy-lint	8m 47s
	41m 26s

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