nixos/gitea: set umask for secret creation
nixos/gitea: set umask for secret creation

This ensures that newly created secrets will have the permissions
`0640`. With this change it's ensured that no sensitive information will
be word-readable at any time.

Related to #121293.

Strictly speaking this is a breaking change since each new directory
(including data-files) aren't world-readable anymore, but actually these
shouldn't be, unless there's a good reason for it.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs