Instead of:
```
CI / Check coding style
CI / Check spelling
CI / debian | Autotools | unit | signal
CI / debian | Meson | unit+func | libomemo
CI / debian | Meson | unit+func | signal
CI / fedora | Autotools | unit | signal
CI / ubuntu | Autotools | unit | signal
Cygwin / cygwin-build
```
We will sort them into "Style", "Linux" and "Cygwin".
Add a GitHub Actions workflow to build with Cygwin on Windows.
Fixes issues by forcing Cygwin bash for all steps, handling temporary script paths,
stripping CRLF line endings, and enabling igncr.
