IO_BASE: don't try to modify the case of a temporary string in-place, work on a copy

(cherry picked from commit 6a0efe6883)

Fixes https://gitlab.com/kicad/code/kicad/-/issues/20228

common/io/io_base.cpp

@@ -76,7 +76,7 @@ bool IO_BASE::CanReadLibrary( const wxString& aFileName ) const
     {
         const std::vector<std::string>& exts = desc.m_FileExtensions;
 
-        wxString fileExt = wxFileName( aFileName ).GetExt().MakeLower();
+        wxString fileExt = wxFileName( aFileName ).GetExt().Lower();
 
         for( const std::string& ext : exts )
         {