go/tools/releaser upgrade-dep - remove dependency on local time zone

Updating dependencies resulted in unexpected diffs due to an implicit dependency on the local machine's time zone.

After running:
```
$ bazel run //go/tools/releaser -- upgrade-dep -mirror=false all
```

I saw a number of files modified like so:
```
---- a/conformance/internal/conformance_proto/BUILD.bazel 1969-12-31 16:00:00.000000000 -0800
+--- a/conformance/internal/conformance_proto/BUILD.bazel 1969-12-31 19:00:00.000000000 -0500
```

The timestamp is already set explicitly, and ideally the time zone would be too.