ci: switch Go steps to estate gobuilder + S3 build cache (#20)

Why: unify on the estate gobuilder image and share the S3-backed Go
build cache across repos instead of pulling golang:1.25 cold each run.

- Swap build/test/pre-commit/release Go steps to gobuilder:0.1.2-alma9
- Add GOCACHEPROG (go-cache-plugin, absolute cache dir) + S3 cache env
- Source AWS creds from org-level GOCACHE secrets
- Leave lint step on golangci-lint image (binary absent from gobuilder)
- Leave rpm/upload/release steps untouched (no Go toolchain)

Reviewed-on: #20
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

.woodpecker/pre-commit.yaml

@@ -3,7 +3,19 @@ when:
 
 steps:
   - name: pre-commit
-    image: git.unkin.net/unkin/almalinux9-gobuilder:20260606
+    image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/gobuilder:0.1.2-alma9
+    environment:
+      GOCACHE_S3_BUCKET: gocache
+      GOCACHE_S3_REGION: us-east-1
+      GOCACHE_S3_ENDPOINT_URL: https://s3.ceph.unkin.net
+      GOCACHE_S3_PATH_STYLE: "true"
+      GOCACHE_KEY_PREFIX: ci-node-lookup
+      GOCACHE_METRICS: "true"
+      GOCACHEPROG: go-cache-plugin --cache-dir=/tmp/gocache
+      AWS_ACCESS_KEY_ID:
+        from_secret: GOCACHE_AWS_ACCESS_KEY_ID
+      AWS_SECRET_ACCESS_KEY:
+        from_secret: GOCACHE_AWS_SECRET_ACCESS_KEY
     commands:
       - uvx pre-commit run --all-files
     backend_options: