Bump pub to 9bf4289d6fd5d6872a8929d6312bbd7098f3ea9c

Changes:
```
> git log --format="%C(auto) %h %s" c4e9ddc..9bf4289
 https://dart.googlesource.com/pub.git/+/9bf4289d Top-level --colors flag (#3467)
 https://dart.googlesource.com/pub.git/+/54eb1851 Hide --trace in the embedding (#3466)
 https://dart.googlesource.com/pub.git/+/b9751fdb Fix stack-trace expectation (#3460)

```

Diff: https://dart.googlesource.com/pub.git/+/c4e9ddc888c3aa89ef4462f0c4298929191e32b9~..9bf4289d6fd5d6872a8929d6312bbd7098f3ea9c/
Change-Id: I5313721779a5a2cda825316a8ece6ed0cd57260e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/248803
Reviewed-by: Jonas Jensen <[email protected]>
Auto-Submit: Sigurd Meldgaard <[email protected]>
Reviewed-by: Sarah Zakarias <[email protected]>
Commit-Queue: Sigurd Meldgaard <[email protected]>

DEPS

@@ -133,7 +133,7 @@ vars = {
   "ply_rev": "604b32590ffad5cbb82e4afef1d305512d06ae93",
   "pool_rev": "c40cc32eabecb9d60f1045d1403108d968805f9a",
   "protobuf_rev": "3149f6f2d323e11dbcc983b7ac8b3b9e9d686293",
-  "pub_rev": "c4e9ddc888c3aa89ef4462f0c4298929191e32b9",
+  "pub_rev": "9bf4289d6fd5d6872a8929d6312bbd7098f3ea9c",
   "pub_semver_rev": "5c0b4bfd5ca57fe16f1319c581dc8c882e9b8cb2",
   "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",
   "rust_revision": "b7856f695d65a8ebc846754f97d15814bcb1c244",