feat: use CARGO_BUILD_TARGET_DIR env

When building in the presence of a CARGO_BUILD_TARGET_DIR, there is no
target dir created inside the repo, so copying to a location within the
local repo dir fails, hence we need to copy to the actual target dir
that is being used.

wezterm-gui/build.rs

@@ -1,3 +1,5 @@
+use std::path::PathBuf;
+
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
 
@@ -168,7 +170,10 @@ END
             .join("WezTerm.app")
             .join("Contents")
             .join("Info.plist");
-        let dest_plist = repo_dir.join("target").join(profile).join("Info.plist");
+        let build_target_dir = std::env::var("CARGO_BUILD_TARGET_DIR")
+            .and_then(|s| Ok(PathBuf::from(s)))
+            .unwrap_or(repo_dir.join("target").join(profile));
+        let dest_plist = build_target_dir.join("Info.plist");
         println!("cargo:rerun-if-changed=assets/macos/WezTerm.app/Contents/Info.plist");
 
         std::fs::copy(&src_plist, &dest_plist)