chore(docker): wire all workflows to single-source version ARGs

The Dockerfile already centralizes all version pins into top-level ARGs
(GO_VERSION, ALPINE_IMAGE, CROWDSEC_VERSION, EXPR_LANG_VERSION, XNET_VERSION).
This change closes the remaining gaps so those ARGs are the single source of
truth end-to-end:

- nightly-build.yml now resolves the Alpine image digest at build time and
  passes ALPINE_IMAGE as a build-arg, matching the docker-build.yml pattern.
  Previously, nightly images were built with the Dockerfile ARG default and
  without a pinned digest, making runtime Alpine differ from docker-build.yml.

- six CI workflows (quality-checks, codecov-upload, benchmark, e2e-tests-split,
  release-goreleaser, codeql) declared a GO_VERSION env var but their setup-go
  steps ignored it and hardcoded the version string directly. They now reference
  ${{ env.GO_VERSION }}, so Renovate only needs to update one value per file
  and the env var actually serves its purpose.

- codeql.yml had no GO_VERSION env var at all; one is now added alongside the
  existing GOTOOLCHAIN: auto entry.

When Renovate bumps Go, it updates the env var at the top of each workflow and
the Dockerfile ARG — zero manual hunting required.

.github/workflows/codecov-upload.yml

@@ -47,7 +47,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@4b73464bb391d4059bd26b0524d20df3927bd417 # v6
         with:
-          go-version: "1.26.1"
+          go-version: ${{ env.GO_VERSION }}
 
           cache-dependency-path: backend/go.sum