slugbuilder: Ensure buildpack directories are writeable
slugbuilder: Ensure buildpack directories are writeable

This allows buildpacks to write the exports file, which is currently
used to pass environment variables between buildpacks run by the multi
buildpack.

See also heroku/heroku-buildpack-nodejs#152

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>
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