CI: Add GOCACHEPROG to the backend builder if GOCACHEPROG is set on the host (#107256)

pkg/build/daggerbuild/backend/builder.go

@@ -16,6 +16,7 @@ type BuildOpts struct {
 	ExperimentalFlags []string
 	Tags              []string
 	WireTag           string
+	GoCacheProg       string
 	Static            bool
 	Enterprise        bool
 }
@@ -154,6 +155,10 @@ func Builder(
 		WithMountedCache("/root/.cache/go", goBuildCache).
 		WithEnvVariable("GOCACHE", "/root/.cache/go")
 
+	if prog := opts.GoCacheProg; prog != "" {
+		builder = builder.WithEnvVariable("GOCACHEPROG", prog)
+	}
+
 	commitInfo := GetVCSInfo(src, version, opts.Enterprise)
 
 	builder = withCue(builder, src).