Support non-standard executable-find for sandboxes

Use `flycheck-executable-find` to find the `cargo` executable. Flycheck
initializes it to `executable-find` but it can be overridden to use
non-standard executable resolving (e.g. when using sandboxes).

Author: bjpbakker

flycheck-rust.el

@@ -114,7 +114,7 @@ name)."
   (let ((json-array-type 'list))
     (-let [(&alist 'targets targets)
            (with-temp-buffer
-             (call-process "cargo" nil t nil "read-manifest")
+             (call-process (funcall flycheck-executable-find "cargo") nil t nil "read-manifest")
              (goto-char (point-min))
              (json-read))]
       ;; If there is a target that matches the file-name exactly, pick that