fix static asset caching after workspace refactor

[syphar]

the design of the in-progress-builds table was broken after deploy (#3258)

• the BUILD_VERSION and APP_USER_AGENT include the git sha as version identifier
• it is used as cache-buster when serving static assets (style.css and such)
• but the docs_rs_utils subcrate was only recompiled when it was changed itself
• so the version wasn't updated, which means the filenames weren't updated, which means they were cached in the CDN & the browser while they changed in the codebase.

Locally this would have compile time impact due to relinking. Alternatives would have been:

• not use the git sha for cache-busting (probably doing that separately)
• not caching static assets locally.