Bump pub to 0764437088fd58eb7af779ecef66bab40dfcf2e9

Changes:
```
> git log --format="%C(auto) %h %s" 37d0592..0764437
 https://dart.googlesource.com/pub.git/+/07644370 Make gitignore validator use gitignores from repo-root and down. (dart-lang#3169)
 https://dart.googlesource.com/pub.git/+/205ea58c Handle build-versions when reformatting ranges (dart-lang#3170)
 https://dart.googlesource.com/pub.git/+/8f7bfb8c migrate 3 leaf files to null-safety (dart-lang#3168)
 https://dart.googlesource.com/pub.git/+/df5db1f1 Extract some parts of pubspec parsing to a null-safe library. (dart-lang#3160)

```

Diff: https://dart.googlesource.com/pub.git/+/37d05928939b3100e7e55c3dff922651db1de1e1~..0764437088fd58eb7af779ecef66bab40dfcf2e9/
Change-Id: I8d938dbf6c3fba3c3f4d98bbaa5835acfaf132a7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/216020
Reviewed-by: Sarah Zakarias <zarah@google.com>
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>

DEPS

@@ -139,7 +139,7 @@ vars = {
   "pool_rev": "7abe634002a1ba8a0928eded086062f1307ccfae",
   "process_rev": "56ece43b53b64c63ae51ec184b76bd5360c28d0b",
   "protobuf_rev": "c1eb6cb51af39ccbaa1a8e19349546586a5c8e31",
-  "pub_rev": "37d05928939b3100e7e55c3dff922651db1de1e1",
+  "pub_rev": "0764437088fd58eb7af779ecef66bab40dfcf2e9",
   "pub_semver_rev": "a43ad72fb6b7869607581b5fedcb186d1e74276a",
   "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",
   "rust_revision": "b7856f695d65a8ebc846754f97d15814bcb1c244",