fix(ci): make .go-build-cache actually cache Go artifacts

(Kudos to @kkloss's lovely pipeline visualizer for sending me down this rabbit hole)

Previously, the .go-build-cache job pointed GOMODCACHEGOCACHE at paths outside $CI_PROJECT_DIR, which the runner silently skips, so every Go job ran cold.

This MR relocates both caches into $CI_PROJECT_DIR and splits them:

• Modules: one shared, read-only cache seeded by a warm-go-mod-cache job (new prepare stage) so consumers can't overwrite the complete set with a partial one.
• Compiled objects: per-job cache keyed on $CI_JOB_NAME_SLUG, since output is specific to build tags, Go version, and -race.

Pre-these changes:

• Example pipeline
• 5-7 minutes per pipeline.
• Test and build jobs running between 3-5 minutes
  • Every job re-downloads full set of dependencies
  • No cache usage

After these changes:

• Warm example pipeline
• ~2 minutes per pipeline.
• Tests and build jobs <1 minute
  • All jobs use cached module dependencies (seeded if invalidated as an initial job)
  • Build artifacts cached by job name (Archive files)