libgit2: force embedded version

Index: vendor/libgit2-sys-0.17.0+1.8.1/build.rs
--- vendor/libgit2-sys-0.17.0+1.8.1/build.rs.orig
+++ vendor/libgit2-sys-0.17.0+1.8.1/build.rs
@@ -26,6 +26,7 @@ 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;
 
     // Specify `LIBGIT2_NO_VENDOR` to force to use system libgit2.
     // Due to the additive nature of Cargo features, if some crate in the
@@ -49,7 +50,7 @@ The build is now aborting. To disable, unset the varia
     }
 
     // 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 && try_system_libgit2().is_ok() {
         // using system libgit2 has worked
         return;