Bundle OpenJDK 11 with Flatpak

There might have been a bug in my Arch Linux installation of GNOME
Builder. I've now installed it the recommend way:

	flatpak install --from https://flathub.org/repo/appstream/org.gnome.Builder.flatpakref

Fixes #1.

app.briar.gtk.json

@@ -4,7 +4,7 @@
     "runtime-version": "3.32",
     "sdk": "org.gnome.Sdk",
     "sdk-extensions": [
-        "org.freedesktop.Sdk.Extension.openjdk10"
+        "org.freedesktop.Sdk.Extension.openjdk11"
     ],
     "command": "briar-gtk",
     "finish-args": [
@@ -16,8 +16,7 @@
         "--filesystem=~/.config/dconf:ro",
         "--talk-name=ca.desrt.dconf",
         "--env=DCONF_USER_CONFIG_DIR=.config/dconf",
-        "--env=PATH=/app/bin:/usr/bin:/app/jdk/bin",
-        "--env=JAVA_HOME=/app/jdk"
+        "--env=PATH=/app/bin:/usr/bin:/app/jre/bin"
     ],
     "build-options": {
         "cflags": "-O2 -g",
@@ -42,9 +41,7 @@
             "name": "openjdk",
             "buildsystem": "simple",
             "build-commands": [
-                "/usr/lib/sdk/openjdk10/install.sh",
-                "/usr/lib/sdk/openjdk10/installjdk.sh",
-                "/usr/lib/sdk/openjdk10/enable.sh"
+                "/usr/lib/sdk/openjdk11/install.sh"
             ]
         },
         {