nix-shell: restore backwards-compat with old nixpkgs

Basically an attempt to resume fixing #5543 for a breakage introduced
earlier[1]. Basically, when evaluating an older `nixpkgs` with
`nix-shell` the following error occurs:

    λ ma27 [~] → nix-shell -I nixpkgs=channel:nixos-18.03 -p nix
    error: anonymous function at /nix/store/zakqwc529rb6xcj8pwixjsxscvlx9fbi-source/pkgs/top-level/default.nix:20:1 called with unexpected argument 'inNixShell'

           at /nix/store/zakqwc529rb6xcj8pwixjsxscvlx9fbi-source/pkgs/top-level/impure.nix:82:1:

               81|
               82| import ./. (builtins.removeAttrs args [ "system" "platform" ] // {
                 | ^
               83|   inherit config overlays crossSystem;

This is a problem because one of the main selling points of Nix is that
you can evaluate any old Nix expression and still get the same result
(which also means that it *still evaluates*). In fact we're deprecating,
but not removing a lot of stuff for that reason such as unquoted URLs[2]
or `builtins.toPath`. However this property was essentially thrown away
here.

The change is rather simple: check if `inNixShell` is specified in the
formals of an auto-called function. This means that

    { inNixShell ? false }:
    builtins.trace inNixShell
      (with import <nixpkgs> { }; makeShell { name = "foo"; })

will show `trace: true` while

    args@{ ... }:
    builtins.trace args.inNixShell
      (with import <nixpkgs> { }; makeShell { name = "foo"; })

will throw the following error:

    error: attribute 'inNixShell' missing

This is explicitly needed because the function in
`pkgs/top-level/impure.nix` of e.g. NixOS 18.03 has an ellipsis[3], but
passes the attribute-set on to another lambda with formals that doesn't
have an ellipsis anymore (hence the error from above). This was perhaps
a mistake, but we can't fix it anymore. This also means that there's
AFAICS no proper way to check if the attr-set that's passed to the Nix
code via `EvalState::autoCallFunction` is eventually passed to a lambda
with formals where `inNixShell` is missing.

However, this fix comes with a certain price. Essentially every
`shell.nix` that assumes `inNixShell` to be passed to the formals even
without explicitly specifying it would break with this[4]. However I think
that this is ugly, but preferable:

* Nix 2.3 was declared stable by NixOS up until recently (well, it still
  is as long as 21.11 is alive), so most people might not have even
  noticed that feature.

* We're talking about a way shorter time-span with this change being
  in the wild, so the fallout should be smaller IMHO.

[1] https://github.com/NixOS/nix/commit/9d612c393abc3a73590650d24bcfe2ee57792872
[2] https://github.com/NixOS/rfcs/pull/45#issuecomment-488232537
[3] https://github.com/NixOS/nixpkgs/blob/release-18.03/pkgs/top-level/impure.nix#L75
[4] See e.g. the second expression in this commit-message or the changes
    for `tests/ca/nix-shell.sh`.

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