Revert "Set GN's script_executable to "python3""
Revert "Set GN's script_executable to "python3""

This reverts commit 6f0d2bf49458fb1b0fdc44e775e81c903da65b29.

Reason for revert: iOS code signing still requires python2

Original change's description:
> Set GN's script_executable to "python3"
>
> In some environments, GN would end up running scripts with python2.
> This change requires that python3 be in the path, but is the safest
> way to guarantee that all scripts are run correctly.
>
> Change-Id: Ifc2601ff9c9f3705ac7f1057a26a87a61211ff67
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/528337
> Reviewed-by: Kevin Lubick <kjlubick@google.com>
> Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

Change-Id: I0cd53a48da3a0782803cf6cb1f877376462b2df4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/528437
Bot-Commit: Rubber Stamper <d516c4cc2192058ab8dae3d306acbdaf2ce6574f@appspot.gserviceaccount.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia