Force shell scripts to keep their Unix line termination

[glandium]

On Unix systems, running a shell script with DOS line terminations
(CRLF) fails because the system attempts to execute whatever is on
the shebang, including the "\r" preceding the "\n".

For some reason, the rust-src tarball's install.sh started having CRLF
line endings starting June 29 on nightly (betas are affected as well).
This makes the install.sh file they contain non-executable.

The rust code that creates install.sh from install-template.sh doesn't
seem to do anything that would affect line terminations. So it seems
what might be happening is that those tarballs are now created on a
Windows machine, and the CRLFs slip in from autoconversion by git.

This .gitattributes should prevent such autoconversion.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[Mark-Simulacrum]

This should be fixed in today's or tomorrow's nightly -- Rust bug here: rust-lang/rust#62276. If you can still reproduce, please let us know there. For now I'm going to close this since it's presumably not an issue anymore (and any additional required fixes should go into rust-lang/rust anyway).

Thank you!