Fail a touched manifest's module check only for what the change brings (hq ADR 0237)

de-spiegel's and link2pay's manifests already fail the module check on main; without
comparing against the base branch every pull request touching them would fail the gate
for a fault none of them made. The gate's own rule: what was already so is said.

cmd/mesh-builder/main.go

@@ -334,7 +334,7 @@ func checkSpecOf(request link.BuildRequest) builder.CheckSpec {
 	c := request.Check
 	spec := builder.CheckSpec{ID: request.ID, Repository: request.Repository, Ref: request.Ref,
 		Owner: c.Owner, Repo: c.Repo, Number: c.Number, Paths: c.Paths, Beside: map[string]builder.Beside{},
-		Modules: c.Modules, New: c.New, Manifests: c.Manifests, Judge: c.Judge,
+		Modules: c.Modules, New: c.New, Manifests: c.Manifests, Judge: c.Judge, Base: c.Base,
 		Toolchain: builder.ToolchainOf(request.Held), Toolchains: builder.ToolchainsOf(request.Held)}
 	for dir, b := range c.Beside {
 		spec.Beside[dir] = builder.Beside{Repository: b.Repository, Ref: b.Ref}