gogs service: fix encoding of secret key

I was getting a secret key like this:

  [security]
  SECRET_KEY = 7X

Use coreutils base64 instead to get the full 256 bits of randomness.

nixos/modules/services/misc/gogs.nix

@@ -169,7 +169,7 @@ in
         ${optionalString (cfg.useWizard == false) ''
           mkdir -p ${cfg.stateDir}/custom/conf
           cp -f ${configFile} ${cfg.stateDir}/custom/conf/app.ini
-          KEY=$(head -c 16 /dev/urandom | tr -dc A-Za-z0-9)
+          KEY=$(head -c 16 /dev/urandom | base64)
           sed -i "s,#secretkey#,$KEY,g" ${cfg.stateDir}/custom/conf/app.ini
         ''}