This reverts commit 189e6f5e.

After some discussion, it seems better not to bump the major version
number since most of the new features since 2.3 are marked
experimental.

nix develop: Add --redirect flag to redirect dependencies

This is primarily useful if you're hacking simultaneously on a package
and one of its dependencies. E.g. if you're hacking on Hydra and Nix,
you would start a dev shell for Nix, and then a dev shell for Hydra as
follows:

  $ nix develop \
    --redirect .#hydraJobs.build.x86_64-linux.nix ~/Dev/nix/outputs/out \
    --redirect .#hydraJobs.build.x86_64-linux.nix.dev ~/Dev/nix/outputs/dev

(This assumes hydraJobs.build.x86_64-linux has a passthru.nix
attribute. You can also use a store path.)

This causes all references in the environment to those store paths to
be rewritten to ~/Dev/nix/outputs/{out,dev}. Note: unfortunately, you
may need to set LD_LIBRARY_PATH=~/Dev/nix/outputs/out/lib because
Nixpkgs' ld-wrapper only adds -rpath entries for -L flags that point
to the Nix store.

Fix the docs about the new NIX_CONFIG env var

Fixes 'nix develop nixpkgs#qpdfview'.

This was accidentally documented as NIX_OPTIONS.

Add NIX_CONFIG env var for applying nix.conf overrides

This was probably revived in a bad merge.

Make bash non-interactive for nix develop --phase

Fix #3975: Currently if Ctrl-C is pressed during a phase, the interactive subshell
is not exited. Removing --rcfile when --phase is present makes bash
non-interactive

Macos big sur installer fixes

Env vars for ZSH were moved from /etc/zshrc to /etc/zshenv in #3608
to address an issue with zshrc getting clobbered by OS updates, but
/etc/zshenv doesn't exist by default--so *nothing* would get set up
for zsh users unless they already happened to have /etc/zshenv.

Creating these files if they don't exist. Also cut separate creation
of profile.d/nix.sh, which isn't needed now.

Some of the changes in #3788 to support non-systemd Nix installs
don't appear to be aware that the darwin installer exists, which
resulted in some skipped steps and inappropriate instructions.

The move from release.nix to flake.nix appears to have lost some
changes from #3628 / 1c56f18a, leaving
create-darwin-volume.sh out of the release tarball.

Under the assumption that this was just an accident/byproduct of when
flake.nix split off and not intentional, I am restoring those edits.

Fixes #3957. Just runs both forms to minimize moving parts.

As mentioned in previous commit, Big Sur changes the syntax for the
xpath command slightly.

In the process of testing out replacements for these, I noticed a few
small simplification wins.

- xpath -> xmllint: xpath's cli interface changed in Big Sur
  rather than add conditional logic for picking the correct
  syntax for xpath, I'm changing to xmllint --xpath, which
  appears to be consistent across versions I've tested...

- /plist/dict/key[text()='Writable']/following-sibling::true[1]
  doesn't do quite what's expected. It was written to try to
  select a <true /> node paired with the Writable key, but it
  will also select the *next* <true /> node that appears even
  if it was paired with another key.

- I think there's also a logic bug in the conditionals here.
  I'm not sure anyone ever actuall saw it, thanks to the xpath
  bug, though. With the xpath fix, this conditional passes if /nix
  does not exist, / IS writable, and the version is Catalina+.

  I think it meant to test for /nix does not exist, / is NOT
  writable, and the version is Catalina+. I reworked this lightly
  to make it a little clearer at the code level.

Keeping this commit narrow for reviewability, but some of these
conditionals will change in subsequent commits in this PR.

Fixes #3852.

Add some internal documentation for flake support objects.

Switch to Nixpkgs 20.09

checks should be relatively fast, but buildStatic depends on a lot of
stuff that isn't in the binary cache (e.g. musl builds of Git and
Mercurial that we probably don't need since we don't link against
them...).

Flake input changes:

* Updated 'nixpkgs': 'github:NixOS/nixpkgs/3a10a004bb5802d5f23c58886722e4239705e733' -> 'github:NixOS/nixpkgs/ad0d20345219790533ebe06571f82ed6b034db31'

Properly type the derivation and substitution goal maps

doc: nix-shell in pure mode does *not* source user bashrc

Fix substitution loop #3964, #3534