nixos/postfixadmin: fix eval & pin to PHP 7.4

Even though some PHP8 compat fixes seem to be in the release that's
packaged here, it seems as if there are still some minor issues[1].

[1] https://github.com/postfixadmin/postfixadmin/issues/395

nixos/modules/services/mail/postfixadmin.nix

@@ -138,7 +138,7 @@ in
       wantedBy = [ "multi-user.target" ];
       path = [
         pgsql.package
-        pkgs.utillinux
+        pkgs.util-linux
       ];
       script = ''
         set -eu
@@ -177,6 +177,7 @@ in
 
     services.phpfpm.pools.postfixadmin = {
       user = user;
+      phpPackage = pkgs.php74;
       phpOptions = ''
         error_log = 'stderr'
         log_errors = on