updated pinning-nixpkgs.md to have info on how to get the commits and channel names to use

[ellyxir]

i went through first steps, then the language basics. at the end of language basics it had some next steps which took me back to declarative shell evironments. first time around, i didnt pay attention to specifics because i didn't know the language yet. this time around, i expected to understand it fully.

i noticed the fetchTarball url https://github.com/NixOS/nixpkgs/tarball/nixos-24.05 and also the url in the detailed description https://github.com/NixOS/nixpkgs/tarball/2a601aafdc5605a5133a2ca506a34a3a73377247.

i had no idea how to create these. the detailed description has a link to ref-pinning-nixpkgs but this doesn't talk about how to find the commit to use.
i thought it would be useful to add that information.

The next section towards-reproducibility-pinning-nixpkgs does discuss pinning to specific commits which would have been helpful to link to, but i think adding this to the manual would help more paths because even the towards-reproducibility-pinning-nixpkgs document references the manual.

[fricklerhandwerk]

Great find, thanks a lot! I continue in #1080 since I can't modify the source branch for some reason.