gnu: maven-core-bootstrap: Fix build.

* gnu/packages/maven (maven-core-bootstrap)[arguments]:
Add the'add-sisu-shebang phase after unpack.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/maven.scm

@@ -1603,6 +1603,10 @@ generally generated from plugin sources using maven-plugin-plugin.")))
          #:tests? #f
          #:phases
          (modify-phases %standard-phases
+           (add-after 'unpack 'add-sisu-shebang
+             (lambda _
+               (substitute* "sisu.sh"
+                 (("^## T") "#!/bin/sh\n## T"))))
            (add-before 'configure 'chdir
              (lambda _
                ;; Required for generating components.xml in maven-core