gnu: flatpak: Fix binary path when exporting application files.

* gnu/packages/package-management.scm (flatpak): Use FLATPAK_BINARY
environment variable instead of patch.
[native-search-paths]: Add FLATPAK_BINARY.
[source]<patches>: Remove "flatpak-fix-path.patch".
* gnu/packages/patches/flatpak-fix-path.patch: Delete file.
* gnu/local.mk (dist_patch_DATA): De-register it.

Fixes: #438
Change-Id: I094234b1ecf9b2bba6d499a0eae4559231a2f199
Signed-off-by: Danny Milosavljevic <dannym@friendly-machines.com>

gnu/packages/package-management.scm

@@ -2096,7 +2096,6 @@ the boot loader configuration.")
         (base32 "0ajbz8ms4h5nyjr59hv9z8vaimj4f3p51v8idmy14qnbmmjwa2nb"))
        (patches
         (search-patches "flatpak-fix-fonts-icons.patch"
-                        "flatpak-fix-path.patch"
                         "flatpak-fix-icon-validation.patch"
                         "flatpak-unset-gdk-pixbuf-for-sandbox.patch"))))
     (build-system meson-build-system)
@@ -2214,6 +2213,13 @@ cp -r /tmp/locale/*/en_US.*")))
                              libarchive
                              libseccomp
                              libxau))
+    (native-search-paths
+     (list (search-path-specification
+            (variable "FLATPAK_BINARY")
+            (separator #f)
+            (files '("bin"))
+            (file-pattern "^flatpak$")
+            (file-type 'regular))))
     (home-page "https://flatpak.org")
     (synopsis "System for building, distributing, and running sandboxed desktop
 applications")