This is used to determine the dependency tree of impure libraries so nix
knows what paths to open in the sandbox.  With the less restrictive
defaults it isn't needed anymore.

Nix now runs builds with a pseudo-terminal to enable colored build
output.

Sadly 10.15 changed /bin/sh to a shim which executes bash, this means it
can't be used anymore without also opening up the sandbox to allow bash.

    Failed to exec /bin/bash as variant for /bin/sh (1: Operation not permitted).

installer: also test for xz to unpack

After installing Nix, I found that all the files and directories
initially copied into the store were writable, with mode 644 or 755:

  drwxr-xr-x 9 root root 4096 Dec 31  1969 /nix/store/ddmmzn4ggz1f66lwxjy64n89864yj9w9-nix-2.3.3

The reason is that that's how they were in the unpacked tarball, and
the install-multi-user script used `rsync -p` without doing anything
else to affect the permissions.

The plain `install` script for a single-user install takes care to
do a `chmod -R a-w` on each store path copied.  We could do the same
here with one more command; or we can pass `--chmod` to rsync, to
have it write the files with the desired modes in the first place.

Tested the new `rsync` command on both a Linux machine with a
reasonably-modern rsync (3.1.3) and a Mac with its default, ancient,
rsync 2.6.9, and it works as expected on both.  Thankfully the latter
is just new enough to have `--chmod`, which dates to rsync 2.6.7.

darwin sandbox

installer: Set files read-only when copying into store