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

ober

7804a4f60d431b44afb86d0f3460485d23e563b9

diff --git a/.builds/ci.yml b/.builds/ci.yml
index cc84678..0fe1e7c 100644
--- a/.builds/ci.yml
+++ b/.builds/ci.yml
@@ -24,7 +24,7 @@ artifacts:
 tasks:
 - build-jerboa: |
     cd jerboa
-    make jerboa
+    make jerboa CHEZ_CONFIGURE_EXTRA=--disable-x11
     echo 'export PATH="$HOME/jerboa/dist:$PATH"' >> ~/.buildenv
 - setup-rust: |
     # Install Rust toolchain matching jerboa's native FFI build.