Move helperfunctions and stdenvAdapters under pkgsFun.
Move helperfunctions and stdenvAdapters under pkgsFun.

The `helperFunctions` and `stdenvAdapters` both use the `pkgs` attribute as
input, either to inherit some properties, either to use it as argument.

The `pkgs` binding used in both expressions of the `helperFunctions` and
`stdenvAdapters` is no longer the result of the `applyGlobalOverrides`
function, but the argument of the `pkgsFun` function.

The `pkgsFun` functions is called twice under `applyGlobalOverrides`, and in
both cases, the first argument of `pkgsFun` correspond to the result of
`applyGlobalOverrides`.

Thus, this modification will change the bindings, but the evaluation of
`<nixpkgs>`.

A third call the `pkgsFun` exists under `overridePackages` in the set of all
packages. Previously, the `helperFunctions` and `stdenvAdapaters` would use
the functions defined as part of the default `<nixpkgs>` set.  With this
modification, the `helperFunctions` and the `stdenvAdapters` are now using
the fix-point of the newly evaluated package set.

This implies that this modification allow the user to use
`overridePackages`, which is already not recommended for performance
reasons, to override the inputs of the `helperFucntions` and
`stdenvAdapaters` too, where this was not possible before.

SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs