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

ober

bddd7dd094f2783e817cf1893508810d43056c1c

diff --git a/.builds/ci.yml b/.builds/ci.yml
index 9864500..1bbac02 100644
--- a/.builds/ci.yml
+++ b/.builds/ci.yml
@@ -22,7 +22,7 @@ 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-site