chore: Bump intra-repo deps to v0.12.6

[mojatter]

Summary

• Prepares the v0.12.6 patch release: bumps the intra-repo github.com/mojatter/s2 (and, for s2env, s3/gcs/azblob) require lines in the five replace-modules to v0.12.6.
• v0.12.5's root tag was abandoned unreleased (see fix(ci): Point release workflow's Set up Go at go.work #166): its GoReleaser run failed because release.yaml installed Go per cmd/s2-server/go.mod (1.25.8) instead of go.work (1.26). fix(ci): Point release workflow's Set up Go at go.work #166 fixes that; this PR re-cuts the release under v0.12.6.
• cmd/s2-server is intentionally excluded here; it's bumped in a follow-up PR after the submodule tags are published (see docs/releasing.md).

Test plan

• go vet ./... and go test ./... at the workspace root
• go test ./... in s3, gcs, azblob, s2env, server, cmd/s2-server
• GOWORK=off go build . in cmd/s2-server (against the previous published version)
• GOWORK=off go build ./... in server
• GOWORK=off go test -tags integtest -c in s3