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

ober

29da92f5f1457acf8234cc309861466143ebc485

diff --git a/.builds/ci.yml b/.builds/ci.yml
index 00a3d6f..f16f197 100644
--- a/.builds/ci.yml
+++ b/.builds/ci.yml
@@ -19,7 +19,7 @@ sources:
 tasks:
 - build-jerboa: |
     cd jerboa
-    make jerboa
+    make jerboa CHEZ_CONFIGURE_EXTRA=--disable-x11
 - verify: |
     cd jerboa-gerbil
     PATH="$HOME/jerboa/dist:$PATH" JERBUILD="$HOME/jerboa/dist/jerbuild" make test