Fix determination of wasi-sdk version when built as a subproject.

version.py

@@ -14,7 +14,7 @@
 GIT_REF_LEN = 12
 
 
-def exec(command, cwd=None):
+def exec(command, cwd=os.path.dirname(sys.argv[0])):
     result = subprocess.run(command, stdout=subprocess.PIPE,
                             universal_newlines=True, check=True, cwd=cwd)
     return result.stdout.strip()