gnu: bazaar: Use 'modify-phases'.

* gnu/packages/version-control.scm (bazaar)[arguments]: Use
'modify-phases'.

gnu/packages/version-control.scm

@@ -94,12 +94,13 @@
      `(#:tests? #f ; no test target
        #:python ,python-2   ; Python 3 apparently not yet supported, see
                             ; https://answers.launchpad.net/bzr/+question/229048
-       #:phases (alist-cons-after
-                 'unpack 'fix-mandir
+       #:phases
+       (modify-phases %standard-phases
+         (add-after 'unpack 'fix-mandir
            (lambda _
              (substitute* "setup.py"
-                     (("man/man1") "share/man/man1")))
-                 %standard-phases)))
+                          (("man/man1") "share/man/man1"))
+             #t)))))
     (home-page "https://gnu.org/software/bazaar")
     (synopsis "Version control system supporting both distributed and centralized workflows")
     (description