proton: Add dotnetfx35.exe builtin override.

proton

@@ -560,6 +560,7 @@ class Session:
         self.env = dict(os.environ)
         self.dlloverrides = {
                 "steam.exe": "b", #always use our special built-in steam.exe
+                "dotnetfx35.exe": "b", #replace the broken installer, as does Windows
                 "mfplay": "n" #disable built-in mfplay
         }