receiver: Fix error message

Signed-off-by: Lewis Marshall <lewis@lmars.net>
receiver: Only append ENV dir if non-empty

Signed-off-by: Lewis Marshall <lewis@lmars.net>
slugbuilder: Fix env_dir when running as slugbuilder user

Fixes #247

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #248 from flynn/slug-build-fixes

Slug build fixes