Add sccache to pythonbuild

[orf]

I was looking at adding this to the actual cpython build, but it looks a bit more complex. I'm not sure how to pass CC and CXX to make as required by the docs, and build-cpython.sh seems to do some checks that could interfere with this:

python-build-standalone/cpython-unix/build-cpython.sh

Line 119 in 78e57bd

elif [ "${CC}" = "musl-clang" ]; then

[indygreg]

I used to have sccache support for this project. I removed it because it didn't yield a meaningful speed-up. The CI for this project is bottlenecked by the volume of CPython build jobs. And individual jobs are frequently I/O bottlenecked from extracting tar archives into containers. There's some CPU bottlenecks compiling. But sccache fails to speed it up materially. I didn't think the complexity was justified.