Don't watch PYTHON_SYS_EXECUTABLE and PATH when unnecessary

[kngwyu]

Fixes #1229.
I confirmed that this works manually, though it's better if @nikhilmitrax can check this.

[kngwyu]

Oops, my first attempt was completely broken...
Please try the force-pushed one.

[nikhilmitrax]

yeah figured. I will test now and respond back. Unfortunately my test code depends on multiple versions of PyO3, so will backport the patch and test.

[nikhilmitrax]

Seems to be working. ✅

Btw, just my 2c,

It might be more ergonomic to check if PYO3_PYTHON is set at the top of the build script, and setting it to the path of python or python3 to the env variable, and then just simply check PYO3_PYTHON for changes.

Either way, this unblocks me so I'm happy with this.

[kngwyu]

It might be more ergonomic to check if PYO3_PYTHON is set at the top of the build script, and setting it to the path of python or python3 to the env variable, and then just simply check PYO3_PYTHON for changes.

But then, if PYO3_PYTHON originates from PATH or PYTHON_SYS_EXECUTABLE, don't we need to check them altogether?

[nikhilmitrax]

I meant something like this:

master...nikhilmitrax:master

(non working code, and the structure definitely needs to be different, but should explain the idea.)

This should achieve the same effect as the original MR, with reduced cargo instructions, and the loop in main()

However, this sort of introduces a magical coupling between find_interpreter() and main(), which I'm not so sure I love. I'll defer to you for what you think is better.

[davidhewitt]

I'm not entirely convinced that'll work for users that are relying on PATH instead of PYO3_PYTHON, as I don't think cargo will know in that case that it needs to rerun when PATH changes.

As we are happy the existing one implementation works, I'm going to merge this as-is and proceed with releasing it in 0.12.2.