Python 3 scripts should be run by python3 executable.

[catull]

On systems with only Python 3 installed, the executable is named python3, not python.

This PR closed #1234.

[catull]

With this change, I can now build a full bootstrap on macOS 15, with LLVM 22 clang and python 3 on board.

[VictorMiasnikov]

It's halph-tested as

2026-08-14T18:31:37.2734998Z Critical Mass Modula-3 version d5.12.1
2026-08-14T18:31:37.2735952Z GitInfo: VictorMiasnikov@3f87f00
2026-08-14T18:31:37.2736956Z https://github.com/VictorMiasnikov/cm3/commits/d5.12.1-ZZYYXX-20260509_22-59
2026-08-14T18:31:37.2737687Z remote: https://github.com/VictorMiasnikov/cm3
2026-08-14T18:31:37.2738232Z revision: 3f87f00
2026-08-14T18:31:37.2738744Z branch: d5.12.1-ZZYYXX-20260509_22-59
2026-08-14T18:31:37.2739213Z last updated: 2022-02-05
2026-08-14T18:31:37.2739544Z compiled: 2026-08-14 18:24:29

https://github.com/VictorMiasnikov/cm3/actions/runs/31824359766