ANGLE build is broken with MSYS2 tools

Commit a218a252c4200cfe7048de81a46f7e48349084d5 introduced a change that made the build call "win_bison" instead of "bison" for the parser generator. That means the standard tool installed by MSYS2 (which is recent enough) is not found.

There doesn't seem to be a good reason to find the tool under a non-standard name. We should rename the tool back to the standard name so that a system-wide installation can be used.