Add environment variable for configuring stock data home

Fixes https://gitlab.com/kicad/code/kicad/-/issues/15687


(cherry picked from commit 9b54537e38)

common/paths.cpp

@@ -169,6 +169,10 @@ wxString PATHS::GetStockDataPath( bool aRespectRunFromBuildDir )
         path = GetExecutablePath() + wxT( ".." );
 #endif
     }
+    else if( wxGetEnv( wxT( "KICAD_STOCK_DATA_HOME" ), &path ) && !path.IsEmpty() )
+    {
+        return path;
+    }
     else
     {
 #if defined( __WXMAC__ )