tectonic.tests: fix the eval

Without the change the eval fails as:

    $ nix build --no-link -f. tectonic.tests
    error:
       … while evaluating the attribute 'biber-compatibility'

       … in the condition of the assert statement
         at pkgs/stdenv/generic/make-derivation.nix:540:1:
          539| # Policy on acceptable hash types in nixpkgs
          540| assert attrs ? outputHash -> (
             | ^
          541|   let algo =

       … in the right operand of the IMPL (->) operator
         at /home/slyfox/dev/git/nixpkgs-master/pkgs/stdenv/generic/make-derivation.nix:540:27:
          539| # Policy on acceptable hash types in nixpkgs
          540| assert attrs ? outputHash -> (
             |                           ^
          541|   let algo =

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: attribute 'outputHashAlgo' missing
       at pkgs/tools/typesetting/tectonic/tests.nix:65:9:
           64|       inherit (emptyFile)
           65|         outputHashAlgo
             |         ^
           66|         outputHashMode