build_fuzzers: allow or default to adding additional docker volume mounts

Currently for go fuzzers (at least), we build everything with no caching. This means downloading all modules again, building from scratch, etc.

It would be nice to mount build/module caches, or at least allow this as an option via environment variable, to speed up builds when using `python infra/helper.py build_image`

Start with python infra/helper.py build_image and trace how build_fuzzers invokes Docker. Check how Go module and build caches are handled during the image build, then verify that additional volume mounts can be enabled by an environment variable or default behavior and that the build completes with the requested caches available.

Written by the indexing model from the issue text.