Handle EPERM when creating a hard link for the chroot

There is a race condition when doing parallel builds with chroots and
the immutable bit enabled.  One process may call makeImmutable()
before the other has called link(), in which case link() will fail
with EPERM.  We could retry or wrap the operation in a lock, but since
this condition is rare and I'm lazy, we just use the existing copy
fallback.

Fixes #9.

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