nix(refactor): Make with_tmp_db true checkedShellScript

This extends the interface of withTmpDb to allow loading different sets
of database fixtures via `postgrest-with-postgresql-xx --fixtures
<path>`.

nix/overlays/checked-shell-script/checked-shell-script.nix

@@ -77,9 +77,8 @@ let
       text =
         ''
           #!${bash_5}/bin/bash
-          set -euo pipefail
-
           source ${argsParser}
+          set -euo pipefail
         ''
 
         + lib.optionalString redirectTixFiles ''