githooks.tests: fix eval

Without the change the eval fails as:

    $ nix build --no-link -f. githooks.tests
    error:
           … while evaluating the 'name' attribute of a derivation

           … while evaluating a branch condition
             at lib/strings.nix:1194:3:
             1193|   # First detect the common case of already valid strings, to speed those up
             1194|   if stringLength string <= 207 && okRegex string != null
                 |   ^
             1195|   then unsafeDiscardStringContext string

           … in the left operand of the AND (&&) operator
             at lib/strings.nix:1194:33:
             1193|   # First detect the common case of already valid strings, to speed those up
             1194|   if stringLength string <= 207 && okRegex string != null
                 |                                 ^
             1195|   then unsafeDiscardStringContext string

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

           error: expected a set but found a string: "githooks-cli"