Allow override of dlls with WINEDLLOVERRIDES environment variable

Proton overrides the environment variable WINEDLLOVERRIDES entirely,
this change allows to add own dll overrides.

proton

@@ -373,7 +373,10 @@ for dll in dlloverrides:
         s = s + ";" + dll + "=" + setting
     else:
         s = dll + "=" + setting
-env["WINEDLLOVERRIDES"] = s
+if "WINEDLLOVERRIDES" in os.environ:
+    env["WINEDLLOVERRIDES"] = os.environ["WINEDLLOVERRIDES"] + ";" + s
+else:
+    env["WINEDLLOVERRIDES"] = s
 
 ARCH_UNKNOWN=0
 ARCH_I386=1