fix sourcemapping so errors show proper stack trace

packages/opencode/script/build.ts

@@ -40,7 +40,7 @@ for (const [os, arch] of targets) {
   await $`tar -xf ../../node_modules/${watcher.replace("@parcel/", "parcel-")}-*.tgz -C ../../node_modules/${watcher} --strip-components=1`
 
   await Bun.build({
-    sourcemap: true,
+    sourcemap: "external",
     compile: {
       target: `bun-${os}-${arch}` as any,
       outfile: `dist/${name}/bin/opencode`,