add cargo watch or bacon to build tools

[ilrudie]

This is to build consensus about adding cargo watch or bacon to our build tools image.

I personally like to work inside of make shell when possible for consistency. I also like using cargo watch and/or bacon to track changes and re-test/re-lint/rebuild automatically.

Upsides:

• handy
• easy to get with native rust tooling cargo install cargo-watch or cargo install --locked bacon

Downsides:

• not strictly required for builds, possibly out of scope as a result
• building native rust tooling like this takes time
• adds ~5mb (bacon) or ~7mb (cargo-watch) to image

[howardjohn]

Can you just install in a volume that is mounted via CONDITIONAL_HOST_MOUNTS? Then you can install the kitchen sink of whatever you want?

[ilrudie]

That's fair. Adding it into the image is not the only way