rustup: Fix for locale bug

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`.

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