Fixed buildLocalCabal by defining temporary $HOME

This fixes build error:

building path(s) ‘/nix/store/c8i7gm134aij9330h77l4sdna635yj0a-foo.nix’
building /nix/store/c8i7gm134aij9330h77l4sdna635yj0a-foo.nix
cabal2nix: /homeless-shelter/.cache: createDirectory: does not exist (No such file or directory)
error: syntax error, unexpected $end, at /nix/store/c8i7gm134aij9330h77l4sdna635yj0a-foo.nix:1:1
(use ‘--show-trace’ to show detailed location information)…
`TEMPDIR` is guaranteed to be set by Nix: <https://github.com/NixOS/nix/blob/master/src/libstore/build.cc#L1632>. I'll fix the `git-annex` expresion to use that variable, too.