fix(legacy_plugin): preserve snap environment variables in crystal pl…

…ugin

This commit fixes an issue where the crystal plugin would fork without snap-related environment variables, which messed up the code path for detecting legacy snapcraft's data files.

snapcraft_legacy/plugins/v2/crystal.py

@@ -96,7 +96,10 @@ def get_build_commands(self) -> List[str]:
         else:
             build_options = ""
 
-        env = dict(LANG="C.UTF-8", LC_ALL="C.UTF-8", SNAP=os.getenv("SNAP"), SNAP_NAME=os.getenv("SNAP_NAME"))
+        # Make sure snap-related environment variables survive through the shell call
+        # Filter environment variables for only the ones beginning with "SNAP"
+        snap_dict = {key: os.environ[key] for key in os.environ if key.startswith("SNAP")}
+        env = dict(LANG="C.UTF-8", LC_ALL="C.UTF-8", **snap_dict)
         env_flags = [f"{key}={value}" for key, value in env.items()]
 
         return [