Explicitly set path to yarn

Summary:
Due to issues with Circle's Docker images ([link](https://twitter.com/circleci/status/974694807091073024)), jobs are failing with an error "yarn not found".

Test Plan
Run on Circle

Release Notes
[GENERAL][MINOR][.circleci] - Fix Circle issue
Closes https://github.com/facebook/react-native/pull/18419

Differential Revision: D7312052

Pulled By: hramos

fbshipit-source-id: 2768b9c69046a2f80518430024d3e7afbbd7de65

scripts/circleci/analyze_code.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+cat <(echo eslint; yarn --silent lint --format=json; echo flow; yarn --silent flow check --json) | GITHUB_TOKEN=$GITHUB_TOKEN CI_USER=$CI_USER CI_REPO=$CI_REPO PULL_REQUEST_NUMBER=$PULL_REQUEST_NUMBER node bots/code-analysis-bot.js
+
+# check status
+STATUS=$?
+if [ $STATUS == 0 ]; then
+  echo "Code analyzed successfully"
+else
+  echo "Code analyzis failed, error status $STATUS"
+fi
+

scripts/circleci/exec_swallow_error.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+# execute command
+$@
+
+# check status
+STATUS=$?
+if [ $STATUS == 0 ]; then
+  echo "Command '$@' completed successfully"
+else
+  echo "Command '$@' exited with error status $STATUS"
+fi
+