fix error when env not there

Author: Jason2866

tools/platformio-build.py

@@ -179,7 +179,10 @@ def add_tinyuf2_extra_image():
 libs = []
 
 variants_dir = join(FRAMEWORK_DIR, "variants")
-build_variants_dir = join(board_config.get("build.variants_dir"))
+try:
+    build_variants_dir = join(board_config.get("build.variants_dir"))
+except:
+    build_variants_dir=""
 
 if "build.variants_dir" in board_config:
     if len(build_variants_dir) > 1: