macOS MacPorts: z3 fails to build if MacPorts Python3 was not installed before

[MSoegtropIMC]

On a brand new Mac with only plain MacPorts (but no ports) installed, building z3 failed when compiling python code.

The issue is that:

• the macOS builtin python3 puts compiled files under /Users/me/Library which is blocked by the opam sandbox
• z3 make compiles python files
• opam requests MacPorts to install python3 and the MacPorts python3 is installed when opam compiles z3, but the shell has the system python3 still in its executable path resolution cache und uses this

Work around: Closing the shell from which coq_platform_make.sh was called, opening a new shell and trying again fixes this.

I guess one has to call hash -r at some point, likely after the depext run.

Note: this happened with zsh as default shell - I have not seen this with bash.

[MSoegtropIMC]

Triage note: @rtetley will test if the problem still persists.