Auto merge of #2610 - japaric:install-from-tempdir, r=alexcrichton

cargo-install: prefer building artifacts in the system temporary directory

and each cargo-install instance creates and uses its own build directory. This
allows running several cargo-install instances in parallel.

If we fail to create a temporary directory for whatever reason fallback to
creating and using a target-install directory in the current directory.

closes #2606

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r? @alexcrichton

Qs:
- Should we preserve the current behavior (`target-install` in `cwd`) as a fallback or remove it and error if we can't create a `TempDir` in `env::temp_dir()`? (we currently error if we can't create `target-install` directory in `cwd`)

- Should I add tests for the issues I raised at #2606? If yes, how can I test `cargo-install` parallelism? Lack of "Blocking waiting for file lock on build directory" in the output of the `cargo` commands? or something else?