libutil: initialize the base64 decode array only once
libutil: initialize the base64 decode array only once

Previously, despite having a boolean that tracked initialization, the
decode characters have been "calculated" every single time a base64
string was being decoded.

With this change we only initialize the decode array once in a
thread-safe manner.

rycee/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix