Allow specifying a path to xtask-downloader via env var

[jgallagher]

#6171 accidentally broke a4x2, because it needs to run xtask download softnpu, which used to directly download softnpu bits and now tries to cargo run --bin xtask-downloader .... cargo isn't available within a4x2 VMs, so this change allows that environment to specify a path to the xtask-downloader binary directly.

oxidecomputer/testbed#60 is the corresponding a4x2 PR to inject xtask-downloader and set the env var.

[sunshowers]

thanks for fixing this!

[leftwo]

Hmm. we may have to update the bench gimlet setup instruction too relating to this.

[jgallagher]

Hmm. we may have to update the bench gimlet setup instruction too relating to this.

Is that https://github.com/oxidecomputer/omicron/blob/main/docs/how-to-run.adoc? At a glance it looks like bench gimlets still use cargo xtask ..., which should be fine, I think?