test: Configure Gitaly's bundler with env vars

Setting up and spawning Gitaly in tests needs to have some Bundler
settings configured. Until now this was done by writing to the local
config file in the ruby directory inside the gitaly dir used in test.

This method didn't seem to be robust for all our user and
use-cases (i.e. CI), because this configuration _could_ be done from
different places.

To make the situation more predictable, this change injects all Bundler
configuration through environment variables instead.

scripts/gitaly-test-build

@@ -13,8 +13,6 @@ class GitalyTestBuild
   include GitalySetup
 
   def run
-    set_bundler_config
-
     # If we have the binaries from the cache, we can skip building them again
     if File.exist?(tmp_tests_gitaly_bin_dir)
       GitalySetup::LOGGER.debug "Gitaly binary already built. Skip building...\n"

scripts/gitaly-test-spawn

@@ -9,7 +9,6 @@ class GitalyTestSpawn
   include GitalySetup
 
   def run
-    set_bundler_config
     install_gitaly_gems if ENV['CI']
     check_gitaly_config!
 

spec/support/helpers/gitaly_setup.rb

@@ -54,9 +54,11 @@ module GitalySetup
     {
       'HOME' => expand_path('tmp/tests'),
       'GEM_PATH' => Gem.path.join(':'),
-      'BUNDLE_APP_CONFIG' => File.join(gemfile_dir, '.bundle'),
       'BUNDLE_INSTALL_FLAGS' => nil,
+      'BUNDLE_PATH' => bundle_path,
       'BUNDLE_GEMFILE' => gemfile,
+      'BUNDLE_JOBS' => '4',
+      'BUNDLE_RETRY' => '3',
       'RUBYOPT' => nil,
 
       # Git hooks can't run during tests as the internal API is not running.
@@ -65,17 +67,11 @@ module GitalySetup
     }
   end
 
-  # rubocop:disable GitlabSecurity/SystemCommandInjection
-  def set_bundler_config
-    system('bundle config set --local jobs 4', chdir: gemfile_dir)
-    system('bundle config set --local retry 3', chdir: gemfile_dir)
-
+  def bundle_path
     if ENV['CI']
-      bundle_path = expand_path('vendor/gitaly-ruby')
-      system('bundle', 'config', 'set', '--local', 'path', bundle_path, chdir: gemfile_dir)
+      expand_path('vendor/gitaly-ruby')
     end
   end
-  # rubocop:enable GitlabSecurity/SystemCommandInjection
 
   def config_path(service)
     case service