`nix-direnv` depends on `nixUnstable` for flakes support, but that
causes it to print a warning unless your user is trusted or the
`experimental-features` setting is enabled. Neither of these are
desirable (note that setting `experimental-features` in nix.conf causes
warnings on `nixStable`).
Since flakes are experimental, this commit makes `nixStable` the default
and optionally allows `nixUnstable` with a new flag `enableFlakes`,
prioritising a good experience for users not using experimental
features.
(cherry picked from commit 03310df843a344215b45b2e8aef11ae9402a40e2)
60f31b6692