test_python_executable failure on macOS 12.7.5

Test added in #120006 fails when running in my private buildbots.

Since it passed in CI, I suspect this means it was relying on the machine configuration or the presence of the source tree to get through the tests. The CI machine was running macOS-13.6.7-x86_64-i386-64bit-Mach-O and mine is macOS-12.7.5-x86_64-i386-64bit-Mach-O.

Mine also applies some install_name_tool modifications to the binaries and deploys them to a new machine before running tests, but given every other test passes and the failure here is to locate a Python module rather than a native one, I have to assume that's not impactful.

stderr:
---
Could not find platform independent libraries <prefix>
Could not find platform dependent libraries <exec_prefix>
Fatal Python error: Failed to import encodings module
Python runtime state: core initialized
ModuleNotFoundError: No module named 'encodings'

Current thread 0x0000000115121600 (most recent call first):
  <no Python frame>
---

I'm not in a position to test or debug, unfortunately. I see that Ned proposed to deprecate this variable entirely in #91057, so I wonder how badly we need this test?

Start with the test_python_executable test added in #120006 and reproduce it on the macOS 12.7.5 build described in the report. Compare its behavior with the macOS 13.6.7 CI environment, including the reported deployment and install_name_tool differences. Done means identifying the cause of the missing encodings module and establishing whether the test or its assumptions need to change.