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

Fixes #3852.

- 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.

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.

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

The move from release.nix to flake.nix appears to have lost some
changes from #3628 / 1c56f18, 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.

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.

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.

Macos big sur installer fixes