Chore: use env in devkit.sh shebang

[euonymos]

This improved portability, particularly, makes it run on NixOS.

More info: https://unix.stackexchange.com/questions/29608/why-is-it-better-to-use-usr-bin-env-name-instead-of-path-to-name-as-my

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[satran004]

Thanks @euonymos
Let me do a quick test on different supported environments before merging this change.