Revert 'gnu: rust: Remove unused "ar" setting.'.

Some versions of Rust do need it and others complain about it.
For now, revert the removal entirely.

This reverts commit ec944a1de9.

gnu/packages/rust.scm

@@ -574,6 +574,7 @@ (define-public rust-1.20
 llvm-config = \"" llvm "/bin/llvm-config" "\"
 cc = \"" gcc "/bin/gcc" "\"
 cxx = \"" gcc "/bin/g++" "\"
+ar = \"" binutils "/bin/ar" "\"
 jemalloc = \"" jemalloc "/lib/libjemalloc_pic.a" "\"
 [dist]
 ") port)))