Would it be better/worse to use stdenv's passthru for this instead of a // update?

I.e. passthru = curPkgs

From my testing this seems to do the exact same thing. I feel like passthru is a slightly nicer alternative to avoid potentially attrset shadowing from //, not that mkShellNoCC and pkgs have anything in common now, but maybe in the future?

They're similar, except for two differences:

• the effect of pkg // foo is not preserved in the return value of pkg.overrideAttrs or any other such functions that re-evaluate the package function
• // is simple and not subject to change

For these reasons I would

• use passthru in cases where the new attributes relate strongly to the package (in this case the shell) and if the new attrs should be preserved in the return value of overrideAttrs
• use // in cases where the addition is circumstantial. Another example would be testing a static build of a package in the tests attribute. Using passthru for that would create an expectation that overrides are applied to that test, but they're not, so it'd be better for such a tests test to be omitted after overrideAttrs.

In this case, calling overrideAttrs only needs to produce a shell, and not the whole package set, so we don't need passthru.
I also think // is nicer because it makes it clear which attrset wins when there's a conflict; the shell, and we don't need to check how mkDerivation works or test it to confirm.