add ZIG_GLOBAL_CACHE_DIR env var

Author: andrewrk

src/main.zig

@@ -718,7 +718,7 @@ fn buildOutputType(
     var test_filter: ?[]const u8 = null;
     var test_name_prefix: ?[]const u8 = null;
     var override_local_cache_dir: ?[]const u8 = try optionalStringEnvVar(arena, "ZIG_LOCAL_CACHE_DIR");
-    var override_global_cache_dir: ?[]const u8 = null;
+    var override_global_cache_dir: ?[]const u8 = try optionalStringEnvVar(arena, "ZIG_GLOBAL_CACHE_DIR");
     var override_lib_dir: ?[]const u8 = try optionalStringEnvVar(arena, "ZIG_LIB_DIR");
     var main_pkg_path: ?[]const u8 = null;
     var clang_preprocessor_mode: Compilation.ClangPreprocessorMode = .no;