Be more lenient when realisations have a conflicting dependency set

- This can legitimately happen (for example because of a non-determinism
  causing a build-time dependency to be kept or not as a runtime
  reference)
- Because of older Nix versions, it can happen that we encounter a
  realisation with an (erroneously) empty set of dependencies, in which
  case we don’t want to fail, but just warn the user and try to fix it.

Forward the whole Nix config to the post-build-hook

Fill `NIX_CONFIG` with the value of the current Nix configuration before
calling the post-build-hook.
That way the whole configuration (including the possible
`experimental-features`, a possibly `--store` option or whatever) will
be made available to the hook

nlohmann_json: Update to 3.9.1, fix use of internal copy

libexpr: Fix read out-of-bound on the heap

Signed-off-by: Pamplemousse <xav.maso@gmail.com>

Add a `devShells` attribute to the flake schema

Make `nix develop .#foo` search `.#devShells.${system}.foo` first

fixed-output derivations: fix incorrect responses for getpwuid

Passing nscd socket into the build environment causes unexpected behavior in programs that make getpwuid and other related calls.

relevant threads:
- https://github.com/NixOS/nix/issues/4991
- https://discourse.nixos.org/t/haunted-nix-build-breaks-isolation/13869

toString also coerces a set with an outPath attribute to a string

 Conflicts:
        src/nix/develop.cc
        src/nix/get-env.sh
        tests/shell.nix

'--delete-older-than 10' deletes the generations older than a single day, and '--delete-older-than 12m' deletes all generations older than 12 days.

This changes makes it throw on those invalid inputs, and gives an example of a valid input.

nix-repl> builtins.toString { outPath = "somestring"; }
"somestring"

nix print-dev-env: Add --json flag

nix develop: Make bash environment parsing more robust

Instead have get-env.sh dump the bash environment as JSON. This should
be a lot less error-prone.

Fixes #4992.

Fix the `name` attribute in builtins.fetchGit

Accidentally broken by 7e5c79a2

Add a name attribute to the fetchers

We need to support it for the “old” fetch* functions for backwards
compatibility, but we don’t need it for fetchTree (as it’s a new
function).
Given that changing the `name` messes-up the content hashing, we can
just forbid passing a custom `name` argument to it

Apply OS checks to host platform, not build

flake.nixConfig: fix flake-registry config settings