UV_FROZEN env var please

[samuelcolvin]

uv is great, thank you!

It would be great if uv respected a UV_FROZEN env var with the same behaviour as --frozen.

This would match UV_NO_SYNC.

It would be particularly helpful in CI where:

1. I could set UV_FROZEN globally rather than adding it to every line
2. I could tell uv to run in "frozen" mode even when commands are withing makefiles or bash scripts, where it's not possible to set the --frozen cli arg

[charliermarsh]

This makes sense, thanks!