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

ober

945caae12e8dd95e9fbc2ab025aa321525b92b87

diff --git a/.builds/ci.yml b/.builds/ci.yml
index 5033436..c777ab0 100644
--- a/.builds/ci.yml
+++ b/.builds/ci.yml
@@ -23,7 +23,7 @@ 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.