pyoxidizer: avoid pyo3 recompiles when Python config has not changed

[dae]

Whenever the pyo3 config file has its modification time bumped, the
pyo3 crates and python-oxidized-importer get recompiled, which adds
to the build time. This change brings an incremental 'pyoxidizer build'
time down from ~24 seconds to ~10 seconds.

This only makes sense in conjunction with something like #468, as
otherwise all Rust crates get recompiled each time anyway.

[dae]

Presented mainly for discussion - I will probably end up going with #466 for now instead, and this doesn't help there, as the artifacts don't seem to get regenerated by run-build-script if they're not removed first. I ended up with a different workaround of copying the generated pyo3 config into a separate folder, and preserving its mtime when it was unchanged, then pointing PYO3_CONFIG_FILE to that other location. It allows me to rebuild the artifacts each time, without the extra crate rebuilding. Compared to 'pyoxidizer build', an incremental build is ~8s.