gnu: pybitmessage: Don't replace 'sh' reference with 'bash'.

* gnu/packages/messaging.scm (pybitmessage)[arguments]: Use (which "sh") instead
of (which "bash") in 'fix-makefile' phase.

gnu/packages/messaging.scm

@@ -825,7 +825,7 @@ connect with friends and family without anyone else listening in.")
                (("mkdir -p \\$\\{DESTDIR\\}/usr") "")
                (("/usr/local") "")
                (("/usr") "")
-               (("#!/bin/sh") (string-append "#!" (which "bash")))
+               (("#!/bin/sh") (string-append "#!" (which "sh")))
                (("python2") (which "python"))
                (("/opt/openssl-compat-bitcoin/lib/")
                 (string-append (assoc-ref inputs "openssl") "/lib/")))