Add GHDL_PREFIX to bin/yosys

[mkj]

With yosys-ghdl-plugin built as a library it doesn't get detected with strings, so instead we can enable GHDL_PREFIX explicitly.

Note that I haven't tested this build locally.

[mmicko]

Thanks for pointing out to issue. Will need to do it a bit different, since it should apply to linux only if ghdl plugin is packaged (which is not case for all linuxes for example), so will update that myself and close this PR later

[mmicko]

Fixed with https://github.com/YosysHQ/oss-cad-suite-build/commit/e8340561ca7c1f593400c9a2ef4ff6a6cc0521c4

[mkj]

Thank you @mmicko