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

ober

bdbb52f68b2134b1fd4ee239e546490fbe17dae7

diff --git a/.build.yml b/.build.yml
index 3ec8a96..9e62c7c 100644
--- a/.build.yml
+++ b/.build.yml
@@ -15,7 +15,7 @@ artifacts:
 tasks:
   - build-jerboa-tools: |
       cd jerboa
-      make jerboa
+      make jerboa CHEZ_CONFIGURE_EXTRA=--disable-x11
       echo 'export PATH="$HOME/jerboa/dist:$PATH"' >> ~/.buildenv
   - test-shell: |
       cd jerboa-shell