rustup: Fix for locale bug

[ghost]

Since tr converts lowercase to uppercase according to system locale using LC_CTYPE environment variable; on some locales, rustup.sh fails to use correct variables names, thus deletes temporarily downloaded files and gives a meaningless error as shown below. This a simple fix which explictly sets LC_CTYPE as C.

Here is what happens without the fix:

➜  projects  curl -s https://static.rust-lang.org/rustup.sh | sudo sh
rustup: CFG_CURL             := /usr/bin/curl (7.22.0)
rustup: CFG_TAR              := /bin/tar (1.26)
rustup: CFG_FILE             := /usr/bin/file (5.09)
rustup: CFG_SHA256SUM        := /usr/bin/sha256sum (256sum)
rustup: CFG_SHASUM           := /usr/bin/shasum (5.61)
rustup: 
rustup: processing sh args
rustup: 
rustup: CFG_PREFiX           :=  
rustup: CFG_DATE             :=  
rustup: 
rustup: validating sh args
rustup: 
rustup: host triple: i686-unknown-linux-gnu
rustup: Downloading https://static.rust-lang.org/dist/rust-nightly-i686-unknown-linux-gnu.tar.gz to /tmp/tmp.Wz6F1ToG5z/rust-nightly-i686-unknown-linux-gnu.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  132M  100  132M    0     0  59947      0  0:38:31  0:38:31 --:--:-- 71204
rustup: Downloading https://static.rust-lang.org/dist/rust-nightly-i686-unknown-linux-gnu.tar.gz.sha256
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   109  100   109    0     0    107      0  0:00:01  0:00:01 --:--:--   169
rustup: Verifying hash
rustup: Extracting /tmp/tmp.Wz6F1ToG5z/rust-nightly-i686-unknown-linux-gnu.tar.gz
install: looking for install programs
install: 
install: found mkdir
install: found printf
install: found cut
install: found grep
install: found uname
install: found tr
install: found sed
install: found chmod
install: 
install: processing /tmp/tmp.Wz6F1ToG5z/rust-nightly-i686-unknown-linux-gnu/install.sh args
install: 
install: CFG_DESTDiR          :=  
install: CFG_PREFiX           := /usr/local 
install: CFG_LiBDiR           := /lib 
install: CFG_MANDiR           := /share/man 
install: 
install: validating /tmp/tmp.Wz6F1ToG5z/rust-nightly-i686-unknown-linux-gnu/install.sh args
install: 
install: verifying platform can run binaries
install: verifying destination is writable
mkdir: cannot create directory `': No such file or directory
install: error: can't write to destination. consider `sudo`.
rustup: error: failed to install Rust

Notice how i wasn't replaced with I.

Rust is installed as usual after the fix. Tested on Ubuntu x86 12.04 LTS.
I'm not exactly sure if setting LC_CTYPE is the best solution, but there's that.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pcwalton (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. 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 CONTRIBUTING.md for more information.

[alexcrichton]

r? @brson

[brson]

@bors: r+ c24f353 rollup

Thanks!