Add PRE_BUILD EnvVar

What does this MR do?

Exposes compilation flag as ENV variable to allow standard interface for specifying build options.

This name was chosen to provide a more common strategy across analyzers, where-as some involve compilation and others involve fetching dependencies.

See gitlab-org/gitlab#8212 (closed) for details and related discussion: gitlab-org/gitlab!19647 (comment 240464144)

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• [-] Documentation created/updated for this project, if necessary
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template (also in CE)
  • Job definition example
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer

/label groupstatic analysis devopssecure ~feature Category:SAST