fix: check for both python3 and python in exec-env

[adamfeldman]

This fix enables the use of Homebrew-installed Python with this plugin.

See also 9d7ff92 for an earlier commit that adjusted python --> python3.

Description

Changes bin/exec-env to check for python3 instead of python.

Motivation and Context

Not working, on macOS with Homebrew-installed Python:

➜ ~ gcloud --version
🚨 Python not found and is required for gcloud. Might I suggest https://github.com/danhper/asdf-python
➜ ~ command -v python
➜ ~
➜ ~ command -v python3
/opt/homebrew/bin/python3

An alias doesn't fix it either:

➜ ~ command -v python
alias python=python3
➜ ~ command -v python3
/opt/homebrew/bin/python3
➜ ~ gcloud help
🚨 Python not found and is required for gcloud. Might I suggest https://github.com/danhper/asdf-python
➜ ~ which python
python: aliased to python3
➜ ~ which python3
/opt/homebrew/bin/python3

After applying the change, it works:

➜ ~ gcloud --version
Google Cloud SDK 433.0.0
bq 2.0.93
core 2023.05.26
gcloud-crc32c 1.0.0
gsutil 5.24

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Usage examples

How Has This Been Tested?

Checklist:

• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[jthegedus]

Thanks for the PR @adamfeldman

I made a change so that it prefers python3 but if it is not found falls back to python.

[adamfeldman]

Awesome, thank you @jthegedus!

[jthegedus]

Sorry for the delay 😅

[adamfeldman]

You were plenty quick! Thank you for maintaining this! 😄 🚀