Fix chroots builds

Chroots are initialised by hard-linking inputs from the Nix store to
the chroot.  This doesn't work if the input has its immutable bit set,
because it's forbidden to create hard links to immutable files.  So
temporarily clear the immutable bit when creating and destroying the
chroot.

Note that making regular files in the Nix store immutable isn't very
reliable, since the bit can easily become cleared: for instance, if we
run the garbage collector after running ‘nix-store --optimise’.  So
maybe we should only make directories immutable.

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