Link the libghostty-vt archive installed by dev-libs/libghostty-vt when
GHOSTTY_VT_LIB_DIR names its directory, instead of cloning Ghostty and
running zig on it. The pin check between ghostty-revision and the
generated ffi.rs stays: it is what ties the bindings to that snapshot.

--- a/build.rs
+++ b/build.rs
@@ -17,6 +17,13 @@
         Some(revision),
         "regenerate the bindings with script/ghostty-bindings"
     );
+    if let Some(dir) = env::var_os("GHOSTTY_VT_LIB_DIR") {
+        // A prebuilt libghostty-vt.a of the pinned revision.
+        let dir = PathBuf::from(dir);
+        println!("cargo:rustc-link-search=native={}", dir.display());
+        println!("cargo:rustc-link-lib=static=ghostty-vt");
+        return;
+    }
     let out = PathBuf::from(env::var_os("OUT_DIR").expect("OUT_DIR"));
     let source = match env::var_os("GHOSTTY_SOURCE_DIR") {
         Some(path) => {