test: fix php-cs-fixer by adding it directly to docker (GoogleCloudPlatform#1091)

Author: bshaffer

.kokoro/docker/php73/Dockerfile

@@ -120,3 +120,5 @@ RUN gcloud config set app/promote_by_default false && \
     gcloud -q components install app-engine-php && \
     gcloud -q components update
 
+# Build php-cs-fixer
+RUN composer global require friendsofphp/php-cs-fixer

.kokoro/system_tests.sh

@@ -52,8 +52,6 @@ export PULL_REQUEST_NUMBER=$KOKORO_GITHUB_PULL_REQUEST_NUMBER
 
 # Run code standards check when appropriate
 if [ "${RUN_CS_CHECK}" = "true" ]; then
-  curl -L https://cs.sensiolabs.org/download/php-cs-fixer-v2.phar -o php-cs-fixer \
-       && chmod a+x php-cs-fixer
   bash testing/run_cs_check.sh
 fi