rustc_wrapper: pass HOMEBREW_RUSTFLAGS last

If we pass it first, it can be overridden by other arguments passed on
the command line. We don't want that.

See discussion at Homebrew/homebrew-core#232566.

Library/Homebrew/shims/super/rustc_wrapper

@@ -12,4 +12,4 @@ shift
 # Prepend HOMEBREW_RUSTFLAGS to the arguments if set.
 # HOMEBREW_RUSTFLAGS is set in extend/ENV/{std,super}.rb
 # shellcheck disable=SC2086,SC2154
-exec "${RUSTC}" ${HOMEBREW_RUSTFLAGS} "$@"
+exec "${RUSTC}" "$@" ${HOMEBREW_RUSTFLAGS}