Use a relative path for the output when building a Dart kernel (#28847)

https://dart.googlesource.com/external/github.com/flutter/engine/+/5f7d0053f98dd5284f69514504e8b0f318ac9f13
diff --git a/commits.json b/commits.json
index efe00ae..6b2975b 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"ed0e6f0a36672e0b0a02dece0d76f2012c9768cd",
+  "external/github.com/flutter/engine":"5f7d0053f98dd5284f69514504e8b0f318ac9f13",
   "external/github.com/flutter/flutter":"b10e1f9a437272a9ce5253661a636e5506a6208a",
   "sdk":"70533ae7fa89c28dffe6ae1e693c124278ff6a34"
 }
\ No newline at end of file