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
23e40d8782