Follow-up from "Update SAST docs for leveraging existing builds"

Description

I'd like to have an env var instead of a parameter, for consistency. Example: gitlab-org/security-products/analyzers/spotbugs!19 (closed)

The following discussion from !19647 (merged) should be addressed:

• @theoretick started a discussion: (+2 comments)

  Finally got around to resurrecting !9676 (closed), I've updated it to exclude some less relevant portions.

  @plafoucriere to address the main blocker previously, -compile=false should indeed be an ENV variable but that's a feature request. A request came up in conversation with @caneldem about this recently so my goal here is to document current state and available workarounds. With the work in #13477 (closed) we can formalize a variable/option to make this universal but frankly I just want this merged so we have something in place when people ask.

Tasks

• Update spotbugs compile flag to also look at an env var named COMPILE gitlab-org/security-products/analyzers/spotbugs!42 (merged)
• Update documentation to suggest usage of COMPILE env var !34211 (merged)
• Add COMPILE env var to documented env vars for spotbugs (this may not be appropriate) !34211 (merged)