fix(nix): Add missing quotes for dev_version if custom value

[erikmd]

Hi, I believe I spotted a bug:

with the meta.yml snippet

tested_coq_nix_versions:
  - coq_version: 'v8.19'
    extra_dev_dependencies:
      - nix_name: mathcomp
        dev_version: 'mathcomp-2.2.0'

I was getting

      matrix:
        overrides:
          - 'coq = "v8.19"; mathcomp = mathcomp-2.2.0'

instead of

      matrix:
        overrides:
          - 'coq = "v8.19"; mathcomp = "mathcomp-2.2.0"'

Cc @Zimmi48 Does my patch look good to you?