Reuse the Go module cache during DEB packaging

[LouisParkin]

DEB packaging passed vendor/ as GOMODCACHE, so Omnibus redownloaded modules despite the populated CI cache. Use go env GOMODCACHE before Omnibus changes GOPATH. This removes the redundant downloads that exposed the spike build to a Go proxy failure.

Validation: offline reproduction fails with the old path and passes with the corrected cache; parsed workflow comparison and offline Zizmor passed. Full packaging CI remains required.

Tracks https://github.com/StackVista/stackstate/issues/561.

[LouisParkin]

Reviewed be48d78 against STAC-25490-otel-spike: no blocking findings.

Traced the argument through tasks/omnibus.py into the installer/agent recipes. Resolving GOMODCACHE before Omnibus overrides GOPATH correctly preserves the populated cache; vendor/ cannot substitute for it when agent.build uses -mod=readonly.

Validation:

• Independent offline reproduction with a ZIP-only module cache and a changed GOPATH: the old path fails, the corrected path builds; go mod verify passes.
• Parsed workflow comparison confirms only the cache argument changes executable behavior. Offline Zizmor reports no findings under the existing configuration.
• CI run 34956835540 built both DEBs, passed branding checks, and built/smoke-tested both container architectures. Logs show /go/pkg/mod in the Omnibus recipes and no subsequent Go download messages. Lint/unit and binary workflows also passed.

The earlier workspace preparation still downloads some modules, so this fixes redundant Omnibus downloads rather than making the whole build offline. PR signing/publication jobs were correctly skipped.