Bump process to 2546dfef7ba839b1514e0c9045344692eb47b771

Changes:
```
> git log --format="%C(auto) %h %s" 56ece43..2546dfe
 https://dart.googlesource.com/process.dart.git/+/2546dfe Make encoding parameter nullable (again) (#68)
 https://dart.googlesource.com/process.dart.git/+/50921c3 Switch to use GitHub Actions instead of Travis, update analysis options (#67)
 https://dart.googlesource.com/process.dart.git/+/6e71d21 Revert "Make encoding parameter nullable (#65)"
 https://dart.googlesource.com/process.dart.git/+/d6c245e Make encoding parameter nullable (#65)
 https://dart.googlesource.com/process.dart.git/+/0c9aeac Add custom exceptions with more information (#63)
 https://dart.googlesource.com/process.dart.git/+/6c873f5 Fix ProcessManager.canRun to be consistent with LocalProcessManager (#62)
 https://dart.googlesource.com/process.dart.git/+/6b2e264 Fix getExecutablePath to only return files that are actually readable and executable. (#61)
 https://dart.googlesource.com/process.dart.git/+/0981bdb Add more details to ArgumentError if process resolution fails (#59)
 https://dart.googlesource.com/process.dart.git/+/7ca6534 Fix ProcessManager signatures to be consistent with LocalProcessManager (#58)

```

Diff: https://dart.googlesource.com/process.dart.git/+/56ece43b53b64c63ae51ec184b76bd5360c28d0b~..2546dfef7ba839b1514e0c9045344692eb47b771/
Change-Id: Icc2fe91324c7f93a48557b36295d1c5817f773d3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/245221
Reviewed-by: Nate Bosch <nbosch@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>

DEPS

@@ -135,7 +135,7 @@ vars = {
   "platform_rev": "1ffad63428bbd1b3ecaa15926bacfb724023648c",
   "ply_rev": "604b32590ffad5cbb82e4afef1d305512d06ae93",
   "pool_rev": "c40cc32eabecb9d60f1045d1403108d968805f9a",
-  "process_rev": "56ece43b53b64c63ae51ec184b76bd5360c28d0b",
+  "process_rev": "2546dfef7ba839b1514e0c9045344692eb47b771",
   "protobuf_rev": "c1eb6cb51af39ccbaa1a8e19349546586a5c8e31",
   "pub_rev": "51435efcd574b7bc18d47a5dd620cb9759dea8f8",
   "pub_semver_rev": "ea6c54019948dc03042c595ce9413e17aaf7aa38",