scikit-misc: add build-scikit-misc.yml for riscv64 wheels

[luhenry]

• Package: scikit-misc
• Version: 0.5.2
• Source: https://github.com/has2k1/scikit-misc
• Docs: https://has2k1.github.io/scikit-misc/stable

Compiles skmisc.loess._loess, a meson-python C/Cython extension that statically links a Fortran LOESS/LINPACK/BLAS core (blas_lite.f, linpack_lite.f, loessf.f) built with gfortran. Upstream publishes no riscv64 wheel.

Mirrors upstream's own build-wheels.yml cibuildwheel job; before-build/test-command come straight from upstream's [tool.cibuildwheel] table (LICENSE staging, skmisc.test()), unchanged.

Differs from upstream

• Nothing beyond the riscv64 image and scoping PIP_ONLY_BINARY to numpy, which has no riscv64 wheel on PyPI (ours does); cython/meson-python build from sdist as upstream's own resolution would too.

Testing

• same as upstream (python -c "import sys; import skmisc; sys.exit(skmisc.test())")

License: OK - BSD-3-Clause, wheel ships upstream's LICENSE; gfortran's GPL runtime vendored by auditwheel is collected by the gpl_sources job.