yosys: Remove flex from run dependencies.
yosys: Remove flex from run dependencies.

Fixes the following warning;
```
WARNING (yosys): dso library package defaults::flex-2.6.4-ha10e3a4_1 in
requirements/run but it is not used (i.e. it is overdepending or perhaps
statically linked? If that is what you want then add it to
`build/ignore_run_exports`)
```

SymbiFlow/conda-packages,SymbiFlow/conda-packages