gnu: rust-parking-lot-core-0.8: Move code changes to snippet.

This allows the changes to propagate through the cargo-build-system.

* gnu/packages/crates-io.scm (rust-parking-lot-core-0.8)[source]: Add
snippet to relax version dependencies.
[arguments]: Remove 'relax-dependencies phase.

gnu/packages/crates-io.scm

@@ -40646,7 +40646,14 @@ synchronization primitives.")
        (file-name (string-append name "-" version ".tar.gz"))
        (sha256
         (base32
-         "08n7w09q6b2prvazbzgwrc9ml7aaf8yg3132ifsayrkwy1nwwzs6"))))
+         "08n7w09q6b2prvazbzgwrc9ml7aaf8yg3132ifsayrkwy1nwwzs6"))
+       (modules '((guix build utils)))
+       (snippet
+        '(begin
+           ;; XXX: The file demands 0.3.60; we have 0.3.56, but
+           ;; that works well, really.
+           (substitute* "Cargo.toml"
+             (("0\\.3\\.60") "0.3.56"))))))
     (build-system cargo-build-system)
     (arguments
      `(#:cargo-inputs
@@ -40658,15 +40665,7 @@ synchronization primitives.")
         ("rust-redox-syscall" ,rust-redox-syscall-0.2)
         ("rust-smallvec" ,rust-smallvec-1)
         ("rust-thread-id" ,rust-thread-id-4)
-        ("rust-winapi" ,rust-winapi-0.3))
-       #:phases (modify-phases %standard-phases
-                  (add-after 'unpack 'relax-dependencies
-                    (lambda _
-                      ;; XXX: The file demands 0.3.60; we have 0.3.56, but
-                      ;; that works well, really.
-                      (substitute* "Cargo.toml"
-                        (("0\\.3\\.60")
-                         ,(package-version rust-backtrace-0.3))))))))
+        ("rust-winapi" ,rust-winapi-0.3))))
     (home-page "https://github.com/Amanieu/parking_lot")
     (synopsis "API for creating custom synchronization primitives")
     (description "This package provides an advanced API for creating custom