Bump pub to 6068f47c264ef790e16411b31b2c94ad6beb1ab6

Changes:
```
> git log --format="%C(auto) %h %s" a949b32..6068f47
 https://dart.googlesource.com/pub.git/+/6068f47c Fixing link example "User-Agent" (#3400)
 https://dart.googlesource.com/pub.git/+/cd0d6992 Make tests more agnostic to the test environment (#3402)
 https://dart.googlesource.com/pub.git/+/1f8db828 Fix retrieval of name in `pub global activate -sgit` (#3407)
 https://dart.googlesource.com/pub.git/+/ed63f212 Limit the number of concurrent precompilations (#3396)
 https://dart.googlesource.com/pub.git/+/d82f80a5 Fix crash when checking code with bad import (#3392)
 https://dart.googlesource.com/pub.git/+/b9ae624e Relative urls in lockfile for git urls (#3385)
 https://dart.googlesource.com/pub.git/+/62bb2440 Fix `dependency_services apply` for packages with relative imports (#3374)

```

Diff: https://dart.googlesource.com/pub.git/+/a949b329b1b51f5f3973a790e0a0a45897d837de~..6068f47c264ef790e16411b31b2c94ad6beb1ab6/
Change-Id: Ib18c2647e27a34d84005214b844351f314698611
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/243321
Commit-Queue: Sigurd Meldgaard <[email protected]>
Reviewed-by: Sarah Zakarias <[email protected]>

Author: sigurdm

DEPS

@@ -138,7 +138,7 @@ vars = {
   "pool_rev": "7abe634002a1ba8a0928eded086062f1307ccfae",
   "process_rev": "56ece43b53b64c63ae51ec184b76bd5360c28d0b",
   "protobuf_rev": "c1eb6cb51af39ccbaa1a8e19349546586a5c8e31",
-  "pub_rev": "a949b329b1b51f5f3973a790e0a0a45897d837de",
+  "pub_rev": "6068f47c264ef790e16411b31b2c94ad6beb1ab6",
   "pub_semver_rev": "ea6c54019948dc03042c595ce9413e17aaf7aa38",
   "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",
   "rust_revision": "b7856f695d65a8ebc846754f97d15814bcb1c244",