gnu: rust-xml-rs-0.8: Fix build.

* gnu/packages/crates-io.scm (rust-xml-rs-0.8)[source]: Add snippet to
change doctests into regular tests.

gnu/packages/crates-io.scm

@@ -27443,7 +27443,14 @@ to XDG Base Directory specification")
         (string-append name "-" version ".tar.gz"))
        (sha256
         (base32
-         "0c905wsmk995xypxljpxzq6vv660r1pzgyrpsfiz13kw3hf0dzcs"))))
+         "0c905wsmk995xypxljpxzq6vv660r1pzgyrpsfiz13kw3hf0dzcs"))
+       (modules '((guix build utils)))
+       (snippet
+        '(begin
+           ;; 'doctest' isn't stable until rust-1.40
+           (substitute* "src/lib.rs"
+             (("\\(doctest") "(test"))
+           #t))))
     (build-system cargo-build-system)
     (arguments
      `(#:cargo-development-inputs