libgit2: force embedded version

Index: vendor/libgit2-sys/build.rs
--- vendor/libgit2-sys/build.rs.orig
+++ vendor/libgit2-sys/build.rs
@@ -9,9 +9,10 @@ fn main() {
     let ssh = env::var("CARGO_FEATURE_SSH").is_ok();
     let vendored = env::var("CARGO_FEATURE_VENDORED").is_ok();
     let zlib_ng_compat = env::var("CARGO_FEATURE_ZLIB_NG_COMPAT").is_ok();
+    let force = true;
 
     // To use zlib-ng in zlib-compat mode, we have to build libgit2 ourselves.
-    let try_to_use_system_libgit2 = !vendored && !zlib_ng_compat;
+    let try_to_use_system_libgit2 = !force && !vendored && !zlib_ng_compat;
     if try_to_use_system_libgit2 {
         let mut cfg = pkg_config::Config::new();
         if let Ok(lib) = cfg.range_version("1.6.4".."1.7.0").probe("libgit2") {