Fix 'sed: RE error: illegal byte sequence' (#1670)

... which is caused by `realpath.dylib` containing illegal UTF-8 byte sequence, and `LC_CTYPE` won't take effect if `LC_ALL` happens to be set to something other than `C`.

This commit fixes issue pyenv/pyenv#1454.

Ref: https://stackoverflow.com/a/23584470

libexec/pyenv-help

@@ -27,7 +27,7 @@ command_path() {
 }
 
 extract_initial_comment_block() {
-  LC_CTYPE=C
+  LC_ALL=C
   LANG=C
   sed -ne "
     /^#/ !{