Sometimes `/tmp` is mounted with the `noexec` option, which results
causes running shebang recipes to fail.
Instead of `/tmp`, use the cache directory as provided by the
[dirs crate](https://docs.rs/dirs/latest/src/dirs/lib.rs.html#77).
Adds an `unindent()` function that strips common leading indentation
from strings, and apply it to integration test case strings, so that they
can be written in a more readable style.
