fix(nix): fix superflous / that breaks the build

[nrdxp]

Resolves #1779

[the-mikedavis]

What was the error you saw from the extra /? I haven't seen the Nix build fail since adding cabal (we have cachix building it in CI as well: https://github.com/helix-editor/helix/actions/runs/4803014728/jobs/8547091610) and #1779 is about the time and caching for fetching grammars rather than an error doing so

[nrdxp]

Seems to be that someone introduced non-deterministic behavior into a deterministic system (nix build) 😅

I can no longer reproduce the build error. It could be that once the /nix/store is popullated with the tree-sitter grammars the error no longer occurs which would also explain why you haven't seen it. I hadn't come back to build from master in quite a while so my /nix/store definitely didn't have anything populated yesterday when I hit the error.

[nrdxp]

@the-mikedavis, by using a temporary /nix/store I was able to reproduce:

❯ n build --store /tmp
[1 copied (137.7 MiB), 145.7 MiB DL] fetching Git repository 'https://gitlab.com/magus/tree-sitter-cabal/'remote:
remote: ========================================================================
remote:
remote: ERROR: The namespace you were looking for could not be found.

remote:
remote: ========================================================================
remote:
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
error:
       … while calling the 'derivationStrict' builtin

         at //builtin/derivation.nix:9:12: (source not available)

       … while evaluating derivation 'helix-term-0.6.0'
         whose name attribute is located at /tmp/nix/store/fb5ci6653i73gr10by36bxg2f5v69lwf-source/pkgs/stdenv/generic/make-derivation.nix:302:7

       … while evaluating attribute 'buildCommand' of derivation 'helix-term-0.6.0'

         at /tmp/nix/store/fb5ci6653i73gr10by36bxg2f5v69lwf-source/pkgs/build-support/trivial-builders.nix:87:14:

           86|       enableParallelBuilding = true;
           87|       inherit buildCommand name;
             |              ^
           88|       passAsFile = [ "buildCommand" ]

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

       error: program 'git' failed with exit code 128

The trace wasn't helpful at all, so I omitted it.