Rework the way the Yosys slang plugin is loaded in `build.mill`. The environment variable
`YOSYS_SLANG_SO` is now accessed with `sys.env.get` to avoid a hard failure when it is not set. If
the variable is present, `-m $YOSYS_SLANG_SO` is passed to Yosys; otherwise the plugin is loaded via
`plugin -i slang`.
