fix(gradle): launch linked builds with the platform's path separators

Linked Gradle builds are saved with forward slashes, so on Windows the launcher of a
build outside the project came out as `C:/…/external\gradlew.bat`. load_build now
rebuilds the build folder from its components, giving the launcher and module folders
native separators; the saved identity is unchanged.

crates/ide-build/src/gradle.rs

@@ -440,7 +440,9 @@ pub fn load(root: &Path) -> Option<GradleModel> {
 
 /// Loads exactly this build, without searching its child folders.
 pub fn load_build(root: &Path, dir: &Path) -> Option<GradleModel> {
-    let dir = root.join(dir);
+    // Linked builds are saved with forward slashes; rebuild the path so the launcher
+    // and module folders use the platform's separators.
+    let dir: PathBuf = root.join(dir).components().collect();
     if !SETTINGS_FILES.iter().chain(BUILD_FILES).any(|name| dir.join(name).is_file()) {
         return None;
     }