Use pub inside the Flutter directory

[liyuqian]

This should fix the version issue in the failed test.

Here's the related error in an example failure log

Executing: pub global activate devtools 0.2.5 in /home/flutter/.cocoon/flutter/dev/benchmarks/complex_layout
--
  | 2020-05-20T13:34:55.628211: stdout: Resolving dependencies...
  | 2020-05-20T13:34:56.302033: stderr: The current Dart SDK version is 2.5.2.
  | stderr:
  | stderr: Because devtools >=0.2.5 depends on devtools_server >=0.1.14 which requires SDK version >=2.6.0 <3.0.0, devtools >=0.2.5 is forbidden.
  | stderr: So, because pub global activate depends on devtools 0.2.5, version solving failed.

[fluttergithubbot]

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[dnfield]

LGTM