build-system/qt: Convert phases to a gexp if needed.

Fixes a bug whereby the "/gnu/store/.*qmlcachegen" string in the
'ktouch' phases would incorrectly be interpreted as a store item,
leading to an error while trying to build it:

  $ ./pre-inst-env guix build ktouch -d --no-grafts
  guix build: error: path ‘/gnu/store/.*qmlcachegen’ is not valid

Reported by Christopher Baines.

* guix/build-system/qt.scm (qt-build)[builder]: If PHASES is a pair,
pass it to 'sexp->gexp'.

guix/build-system/qt.scm

@@ -142,7 +142,9 @@ provides a 'CMakeLists.txt' file as its build system."
                     #:search-paths '#$(sexp->gexp
                                        (map search-path-specification->sexp
                                             search-paths))
-                    #:phases #$phases
+                    #:phases #$(if (pair? phases)
+                                   (sexp->gexp phases)
+                                   phases)
                     #:qt-wrap-excluded-outputs #$qt-wrap-excluded-outputs
                     #:configure-flags #$configure-flags
                     #:make-flags #$make-flags