dockerimg: set GOTOOLCHAIN=auto for cross compiling

Without this, if a user sets GOTOOLCHAIN=local in their host env the
build will fail.
diff --git a/dockerimg/dockerimg.go b/dockerimg/dockerimg.go
index 0e612aa..4135757 100644
--- a/dockerimg/dockerimg.go
+++ b/dockerimg/dockerimg.go
@@ -369,7 +369,13 @@
 
 	start := time.Now()
 	cmd := exec.CommandContext(ctx, "go", "install", "sketch.dev/cmd/sketch@latest")
-	cmd.Env = append(os.Environ(), "GOOS=linux", "CGO_ENABLED=0", "GOPATH="+tmpGopath)
+	cmd.Env = append(
+		os.Environ(),
+		"GOOS=linux",
+		"CGO_ENABLED=0",
+		"GOTOOLCHAIN=auto",
+		"GOPATH="+tmpGopath,
+	)
 
 	fmt.Printf("building linux agent binary...\n")
 	out, err := cmd.CombinedOutput()