nimble file should avoid @#head when possible

[c-blake]

See https://github.com/c-blake/hldiff/issues/1#issuecomment-682113924 & subsequent.

Seems the opengl@#head goes back to Summer 2018 and there have been a great many releases since. Presumably there is one of them that would work better than #head.

[c-blake]

Specifically, the @#head dependency means that on an end-nimble user's pkgs directory, the version will be frozen to their dependency-install-time version - maybe years ago, not the current version control opengl@#head. So, this kind of dependency is just about maximally unreliable in terms of nimble install nimx producing a working install. Personally, I view this as very unintuitive nimble behavior, but it seems not likely to change anytime soon.