Fix HTTPie pygments dependency in Flatpak

Use --ignore-installed to ensure all dependencies (including Pygments)
are installed to the Flatpak prefix, not relying on SDK packages that
won't be available at runtime.

cz.vesp.roster.json

@@ -32,7 +32,7 @@
             "name" : "httpie",
             "buildsystem" : "simple",
             "build-commands" : [
-                "pip3 install --prefix=${FLATPAK_DEST} httpie"
+                "pip3 install --prefix=${FLATPAK_DEST} --ignore-installed httpie"
             ]
         },
         {