show used build image in about/builds view

[syphar]

similar to us showing the used nightly version on https://docs.rs/about/builds ,

we could show the version of crates-build-env that the builder is currently using.

Currently we update the image when we restart the server (mostly when we deploy), or after the scheduled image update.

I'm not sure if we could just store the information somewhere and then show it, or fetch it live from docker locally.

[syphar]

mentoring instructions:

• in reinitialize_workspace_if_interval_passed we refresh our build workspace regularly, which then calls
• build_workspace (also called on server boot), which pulls our builder docker image.
• it would be cool to show a link to the github package registry to the version of that image that we use,
• best case even links to the commit on https://github.com/rust-lang/crates-build-env