Try always using DISPLAY=unix:99:0 on Linux CI

script/cibuild

@@ -61,9 +61,6 @@ def main():
     execute(['sudo', 'apt-get', 'install'] + deps)
 
     execute(['sh', '-e', '/etc/init.d/xvfb', 'start'])
-
-
-  if PLATFORM == 'linux':
     os.environ['DISPLAY'] = ':99.0'
 
   # CI's npm is not reliable.

script/cibuild-linux

@@ -36,6 +36,7 @@ docker run \
   --env ELECTRON_S3_BUCKET="$BUILD_ELECTRON_ELECTRON_S3_BUCKET" \
   --env ELECTRON_S3_ACCESS_KEY="$BUILD_ELECTRON_ELECTRON_S3_ACCESS_KEY" \
   --env ELECTRON_S3_SECRET_KEY="$BUILD_ELECTRON_ELECTRON_S3_SECRET_KEY" \
+  --env DISPLAY="unix:99:0" \
   --user "$UID" \
   --volume "$PWD":/workspace/electron \
   --volume /tmp/.X11-unix:/tmp/.X11-unix  \