Disable X11 in Chez CI bootstrap (no libx11-dev available)

ober

2c07aa7efe891ca6a1331d271dab03a3c9185d07

diff --git a/.builds/ci.yml b/.builds/ci.yml
index a573e46..4707e8c 100644
--- a/.builds/ci.yml
+++ b/.builds/ci.yml
@@ -21,7 +21,7 @@ artifacts:
 tasks:
 - build-jerboa: |
     cd jerboa
-    make jerboa
+    make jerboa CHEZ_CONFIGURE_EXTRA=--disable-x11
     echo 'export PATH="$HOME/jerboa/dist:$PATH"' >> ~/.buildenv
 - package: |
     cd jerboa-mail