Don't lose data when converting wxExecute() arguments under Unix

Use wxConvWhateverWorks when converting the command line given as a string to
individual arguments: we already used wxSafeConvertWX2MB() when converting the
arguments specified as an array, but not here.

Closes #16206.

src/unix/utilsunx.cpp

@@ -400,7 +400,7 @@ public:
 
         for ( int i = 0; i < m_argc; i++ )
         {
-            m_argv[i] = wxStrdup(args[i]);
+            m_argv[i] = wxStrdup(args[i].mb_str(wxConvWhateverWorks));
         }
     }