fix: use original xlings binary (MCPP_VENDORED_XLINGS) on Windows

The copied xlings binary in the mcpp sandbox fails to install large
packages (LLVM 148MB) — likely missing runtime dependencies that the
original xlings has in its own subos structure.

On Windows, when MCPP_VENDORED_XLINGS is set, use the original
binary directly for xlings commands. XLINGS_HOME still points to
the mcpp sandbox, keeping packages self-contained.

Also keep the fallback copy from xlings global data in package_fetcher
as defense-in-depth for cases without MCPP_VENDORED_XLINGS.

Author: Sunrisepeak

src/config.cppm

@@ -84,6 +84,18 @@ struct GlobalConfig {
 
 // Create an xlings::Env from the resolved GlobalConfig.
 mcpp::xlings::Env make_xlings_env(const GlobalConfig& cfg) {
+#if defined(_WIN32)
+    // On Windows, the copied xlings binary in the sandbox may not function
+    // correctly for large package installs (missing runtime environment).
+    // When MCPP_VENDORED_XLINGS is set, use the original xlings binary
+    // directly — it has the full xlings runtime. The XLINGS_HOME env var
+    // ensures packages are installed into the mcpp sandbox.
+    if (auto* e = std::getenv("MCPP_VENDORED_XLINGS"); e && *e) {
+        std::filesystem::path vendored{e};
+        if (std::filesystem::exists(vendored))
+            return { vendored, cfg.xlingsHome() };
+    }
+#endif
     return { cfg.xlingsBinary, cfg.xlingsHome() };
 }