Rollup merge of #22420 - posixphreak:fix-rustup-locale, r=brson

 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.

Author: Manishearth

src/etc/rustup.sh

@@ -241,6 +241,9 @@ create_tmp_dir() {
     echo $TMP_DIR
 }
 
+# Make `tr` locale independent
+LC_CTYPE=C
+
 probe_need CFG_CURL  curl
 probe_need CFG_TAR   tar
 probe_need CFG_FILE  file