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

Fixes this random failure:

    error: hash mismatch in fixed-output derivation '/tmp/nix-shell.EUgAVU/nix-test/tests/check/store/sfps3l3c5n7dabpx34kigxnfhmrwk2h6-dummy.drv':
             specified: sha256-47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
                got:    sha256-0qhPS4tlCTfsj3PNi+LHSt1akRumTfJ0WO2CKdqASiY=

which happens because multiple tests were writing to ./dummy.