gnu: msmtp: Use G-Expressions.

* gnu/packages/mail.scm (msmtp)[arguments]: Rewrite using G-Expressions.

gnu/packages/mail.scm

@@ -1736,14 +1736,16 @@ addons which can add many functionalities to the base client.")
      (list pkg-config))
     (home-page "https://marlam.de/msmtp/")
     (arguments
-     `(#:configure-flags (list "--with-libgsasl"
+     (list
+       #:configure-flags
+       #~(list "--with-libgsasl"
                "--with-libidn"
                "--with-tls=gnutls")
        #:phases
-       (modify-phases %standard-phases
+       #~(modify-phases %standard-phases
            (add-after 'install 'install-additional-files
              (lambda* (#:key outputs #:allow-other-keys)
-             (let* ((out (assoc-ref outputs "out"))
+               (let* ((out #$output)
                       (bin (string-append out "/bin"))
                       (doc (string-append out "/share/doc/msmtp"))
                       (msmtpq "scripts/msmtpq")