Configure Godot Flatpak so that OpenJDK 17 extension installs separately along with it

[Zishan-Rahman]

The reason why #155 stemmed from the need to revert changes made in #151 and #152 is because, while Java is available during build time, it isn't always available afterwards to the end user, who might not have org.freedesktop.Sdk.Extension.openjdk17 installed separately. This PR addresses that by making sure the OpenJDK 17 Flatpak gets installed with the Godot Flatpak, and that the Godot Flatpak can access it.

May fix #157 as the OpenJDK 17 Flatpak includes Gradle binaries.

[flathubbot]

Started test build 103080

[flathubbot]

Build 103080 failed

[Zishan-Rahman]

flatpak run --command=flatpak-builder-lint org.flatpak.Builder --exceptions repo repo
 in dir /srv/buildbot/worker/build-x86_64/build (timeout 1200 secs)
 watching logfiles {}
 argv: b'flatpak run --command=flatpak-builder-lint org.flatpak.Builder --exceptions repo repo'
 using PTY: False
{
    "errors": [
        "appstream-failed-validation"
    ],
    "appstream": [
        "E: org.godotengine.Godot.desktop:~: desktop-app-launchable-omitted"
    ],
    "message": "Please consult the documentation at https://docs.flathub.org/docs/for-app-authors/linter"
}
program finished with exit code 1
elapsedTime=1.395593

[flathubbot]

Started test build 103092

[flathubbot]

Build 103092 failed

[j20001970]

Flatpak docs mentioned that the extensions directory path is relative to /app for applications, JAVA_HOME should be started from /app as well.

[flathubbot]

Started test build 103210

[flathubbot]

Build 103210 failed

[bbhtt]

bot, build

[flathubbot]

Queued test build for org.godotengine.Godot.

[flathubbot]

Started test build 103219

[flathubbot]

Build 103219 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/85999/org.godotengine.Godot.flatpakref