FindANTLR.cmake: should also look in `$CLASSPATH`

The `cmake/modules/FindANTLR.cmake` looks in a few hard-coded well-known locations for a potential antlr*complete.jar.

https://github.com/chipsalliance/Surelog/blob/edb409a1f730d59665c312551aadc0f76af1c82e/cmake/modules/FindANTLR.cmake#L13-L21

However, that is not necessarily where the file is.
It would be good if it was also consulting the `CLASSPATH` (arguably, it even should look there _first_ before going into guess-mode rummaging through directories).

While at it, looking for exactly version 4.12.0 is very specific, can we change that to 4.12.* to be future compatible in case there are new patchlevels coming out ?