diffoscope: fix the eval

Without the change the eval fails as:

    $ nix build --no-link -f. diffoscope
    error:
       … while evaluating the attribute 'drvPath'
         at lib/customisation.nix:416:7:
          415|     // {
          416|       drvPath =
             |       ^
          417|         assert condition;

       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:34:12:
           33|
           34|   strict = derivationStrict drvAttrs;
             |            ^
           35|

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

       error: fetchFromGitHub requires one of either `rev` or `tag` to be provided (not both).

0 files changed, 0 insertions, 0 deletions