[CI] Test failure: `test_android` fails to download dependencies using Buck

[hramos]

Failure first observed in https://app.circleci.com/pipelines/github/facebook/react-native/3569/workflows/1e36a9b4-5bd8-4faa-8baa-ad1eb1093a50/jobs/136206.

Introduced by ee93ee6, which modifies ReactAndroid/src/main/java/com/facebook/fbreact/specs/BUCK:

#!/bin/bash -eo pipefail
./scripts/circleci/buck_fetch.sh
+ buck fetch ReactAndroid/src/test/java/com/facebook/react/modules
Not using buckd because watchman isn't installed.
Picked up _JAVA_OPTIONS: -XX:+UnlockExperimentalVMOptions -XX:+UseCGroupMemoryLimitForHeap
PARSING BUCK FILES: FINISHED IN 1.8s
Buck wasn't able to parse /root/react-native/ReactAndroid/src/main/java/com/facebook/fbreact/specs/BUCK:
KeyError: 'load label @fbsource//tools/build_defs:platform_defs.bzl references an unknown cell named fbsource known cells: {}'
Call stack:
  File "/root/react-native/.buckd/resources/2019.10.17.01/buck_server/buck_parser/profiler.py", line 507, in wrapped
    return func(*args, **kwargs)
  File "/root/react-native/ReactAndroid/src/main/java/com/facebook/fbreact/specs/BUCK", line 1
    load("@fbsource//tools/build_defs:platform_defs.bzl", "ANDROID")


This error happened while trying to get dependency '//ReactAndroid/src/main/java/com/facebook/fbreact/specs:FBReactNativeSpec' of target '//ReactAndroid/src/main/java/com/facebook/react/devsupport:devsupport'

Exited with code exit status 1
CircleCI received exit code 1

Repro

react-native $ ./scripts/circleci/buck_fetch.sh 
+ buck fetch ReactAndroid/src/test/java/com/facebook/react/modules
Guessing 168a69309928ba16065cdb33b1775a4af9f924a6 as the last one used version.
Searching for old Buck versions
Starting new Buck daemon...
Buck daemon started.
Using additional configuration options from /Users/hramos/.buckconfig.d/experiments, /etc/buckconfig.d/fb_chef.ini, /etc/buckconfig.d/fb_chef_override.ini
Parsing buck files: finished in 2.0 sec
Buck wasn't able to parse /Users/hramos/git/react-native/ReactAndroid/src/main/java/com/facebook/fbreact/specs/BUCK:
KeyError: 'load label @fbsource//tools/build_defs:platform_defs.bzl references an unknown cell named fbsource known cells: {}'
Call stack:
  File "/Users/hramos/git/react-native/.buckd/resources/168a69309928ba16065cdb33b1775a4af9f924a6/buck_server/buck_parser/profiler.py", line 507, in wrapped
    return func(*args, **kwargs)
  File "/Users/hramos/git/react-native/ReactAndroid/src/main/java/com/facebook/fbreact/specs/BUCK", line 1
    load("@fbsource//tools/build_defs:platform_defs.bzl", "ANDROID")


This error happened while trying to get dependency '//ReactAndroid/src/main/java/com/facebook/fbreact/specs:FBReactNativeSpec' of target '//ReactAndroid/src/main/java/com/facebook/react/devsupport:devsupport'