Fix how we override CAML_LD_LIBRARY_PATH for opam packages

We need to do this:
```
"CAML_LD_LIBRARY_PATH": {
"val": "#{self.lib / '' : $CAML_LD_LIBRARY_PATH}"
}
```
automatically.

No files or tests are named. Start by locating the code that generates environment overrides for opam packages and search for CAML_LD_LIBRARY_PATH; determine where the shown value should be produced automatically. Done means the override is generated with the package's library path while preserving the existing variable.

Written by the indexing model from the issue text.