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.
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