nix-darwin: fix launchd decrypt scripts

[Mic92]

Untested, potentially fixes #700

please test: @Kidswiss @mettavi

[mettavi]

With this commit, org.nixos.sops-install-secrets (and sops-install-secrets-for-users) is now:

/bin/sh -c '/bin/wait4path /nix/store && exec sh -c \'
echo "Setting up secrets..."

And with the generateKeys option enabled and then rebooting, the secrets are successfully mounted at startup.

It fixed it, thank you @Mic92.

[Kidswiss]

Works for me too, thanks!