third_party/setup.py: fix wabt platform detection on Linux

wabt changed its release asset naming convention in 1.0.39. On Linux,
the old name 'ubuntu' no longer matches the new names 'linux-x64' and
'linux-arm64', causing setup.py to silently fail with an empty download
URL.

Author: BilelGho

third_party/setup.py

@@ -174,13 +174,13 @@ def v8_main():
 
 def wabt_determine_platform():
     if sys.platform.startswith('linux'):
-        return 'ubuntu'
+        return 'linux'
     if sys.platform == 'darwin':
         return 'macos'
     if sys.platform == 'win32':
         return 'windows'
-    print('Cannot determine platform, assuming \'ubuntu\'')
-    return 'ubuntu'
+    print('Cannot determine platform, assuming \'linux\'')
+    return 'linux'
 
 
 def wabt_determine_release(platform):