Revert the behaviour of antiquoted paths to pre-Nix 1.6
Revert the behaviour of antiquoted paths to pre-Nix 1.6

Commit 159e621d1a9c4391b53f3d822109c36931934698 accidentally changed
the behaviour of antiquoted paths, e.g.

  "${/foo}/bar"

used to evaluate to "/nix/store/<hash>-foo/bar" (where /foo gets
copied to the store), but in Nix 1.6 it evaluates to "/foo/bar".  This
is inconsistent, since

  " ${/foo}/bar"

evaluates to " /nix/store/<hash>-foo/bar".  So revert to the old
behaviour.

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