ci: add ccache for riscv64 wheel builds

Install and enable ccache inside the manylinux container to speed up
subsequent C/C++ compilation. First build populates the cache, later
builds benefit from cached object files.

Signed-off-by: Bruno Verachten <gounthar@gmail.com>

Author: gounthar

.github/workflows/wheels.yml

@@ -116,6 +116,8 @@ jobs:
         run: python3 -m cibuildwheel --output-dir wheelhouse
         env:
           CIBW_BUILD: ${{ matrix.python }}-${{ matrix.buildplat[1] }}
+          CIBW_ENVIRONMENT: "CC='ccache gcc' CXX='ccache g++'"
+          CIBW_BEFORE_ALL: "yum install -y ccache || apt-get install -y ccache || true"
 
       - uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f # v7.0.0
         with: