gnu: astromance: Correctly link to game data.

Fixes <http://issues.guix.gnu.org/43797>.

* gnu/packages/games.scm (astromance)[arguments]: Adjust configure-flags
to point the DATADIR to a directory in the output.

gnu/packages/games.scm

@@ -429,7 +429,9 @@ physics settings to tweak as well.")
     (build-system cmake-build-system)
     (arguments
      `(#:tests? #f                      ;no test
-       #:configure-flags '("-DDATADIR=share/astromenace")
+       #:configure-flags (list (string-append "-DDATADIR="
+                                              (assoc-ref %outputs "out")
+                                              "/share/astromenace"))
        #:phases
        (modify-phases %standard-phases
          (replace 'install