Python loads .so using mmap. Rewriting a .so in-place when there is a Python
process with that .so mmap-ed could crash that Python process. Therefore
let's avoid rewriting the destination .so in-place.
This was discovered by a txnclose Python hook I used to keep binaries
up-to-date. It caused segfault after building the Rust extensions currently
loaded by the Python process running the hook.