Fixes #333

Fixes #332

This changes the indentation from tabs to spaces, since tabs get
stripped by the heredoc.

Without this, each time the env script gets executed it will add a copy
of the paths to PATH. This can happen when shells are nested.

The "case" trick was inspired by:
https://github.com/rust-lang/rustup/blob/184a899f4529f27e23eca537a5979c530ff7fa26/src/cli/self_update/env.sh

This reverts commit dbadcf18.