Use deps cache when generating documentation via gleam docs build

[TanklesXL]

The current behaviour for the gleam docs build command is to compile the entire dependency tree from scratch each time.

It would be nice if it used the full build cache, but barring issue #1970 being resolved, that doesnt seem likely.

In the meantime, it would be great if it respected the deps cache so that it only needed to recompiled the current project.

[lpil]

Thank you