Fix assumption in nix buildpack about workdir

repo2docker/buildpacks/nix/install-nix.bash

@@ -5,6 +5,8 @@ set -ex
 NIX_VERSION="2.1.1"
 NIX_SHA256="ad10b4da69035a585fe89d7330037c4a5d867a372bb0e52a1542ab95aec67999"
 
+# Do all our operations in /tmp, since we can't rely on current directory being writeable yet.
+cd /tmp
 wget --quiet https://nixos.org/releases/nix/nix-$NIX_VERSION/nix-$NIX_VERSION-x86_64-linux.tar.bz2
 echo "$NIX_SHA256  nix-2.1.1-x86_64-linux.tar.bz2" | sha256sum -c
 tar xjf nix-*-x86_64-linux.tar.bz2