gnu: sshuttle: Rely on $PATH to find a POSIX sh.

This fixes a regression introduced in commit
c32863e094, where the sshuttle client
will try to invoke its own /gnu/store/.../bin/sh on the server.

Rely on $PATH instead of regressing to upstream's hard-coded /bin/sh.

* gnu/packages/vpn.scm (sshuttle)[arguments]: SUBSTITUTE* "sh" for
"/bin/sh".

gnu/packages/vpn.scm

@@ -355,8 +355,7 @@ (define-public sshuttle
              (substitute* "sshuttle/client.py"
                (("/usr/bin/env") (which "env")))
              (substitute* "sshuttle/ssh.py"
-               ;; Perhaps this is unreachable, but don't let's take risks.
-               (("/bin/sh") (which "sh")))
+               (("/bin/sh") "sh"))
              #t)))))
     (native-inputs
      `(("python-setuptools-scm" ,python-setuptools-scm)