ci: Switch Flatpak build to flathub-infra/flatpak-github-actions (#10838)

* ci: Switch Flatpak build to flathub-infra/flatpak-github-actions

Flathub team has recently forked flatpak-github-actions and merged
various PRs submitted to the original repo. However, it's not versioned
(yet?), so pin the latest commit instead.

Additionally, enable validation of the build using flatpak-builder-lint,
and run all steps in the container with the runtime and required tooling
baked in.

* Update mirror-screenshots-url

* Shush git complaining about "dubious" ownership

* Update date format to iso8601

* Fix flatpak-builder-lint invocations

.github/workflows/scripts/linux/generate-metainfo.sh

@@ -8,7 +8,7 @@ if [[ $# -lt 1 ]]; then
 fi
 
 OUTFILE=$1
-GIT_DATE=$(git log -1 --pretty=%cd --date=short)
+GIT_DATE=$(git log -1 --pretty=%cd --date=iso8601)
 GIT_VERSION=$(git tag --points-at HEAD)
 GIT_HASH=$(git rev-parse HEAD)
 

.github/workflows/scripts/linux/install-packages-flatpak.sh

@@ -1,58 +0,0 @@
-#!/bin/bash
-
-SCRIPTDIR=$(dirname "${BASH_SOURCE[0]}")
-source "$SCRIPTDIR/functions.sh"
-
-set -e
-
-ARCH=x86_64
-KDE_BRANCH=6.6
-BRANCH=23.08
-FLAT_MANAGER_CLIENT_DIR="$HOME/.local/bin"
-
-# Build packages. Mostly needed for flat-manager-client.
-declare -a BUILD_PACKAGES=(
-  "flatpak"
-  "flatpak-builder"
-  "appstream-util"
-  "python3-aiohttp"
-  "python3-tenacity"
-  "python3-gi"
-  "gobject-introspection"
-  "libappstream-glib8"
-  "libappstream-glib-dev"
-  "libappstream-dev"
-  "gir1.2-ostree-1.0"
-)
-
-# Flatpak runtimes and SDKs.
-declare -a FLATPAK_PACKAGES=(
-  "org.kde.Platform/${ARCH}/${KDE_BRANCH}"
-  "org.kde.Sdk/${ARCH}/${KDE_BRANCH}"
-  "org.freedesktop.Platform.ffmpeg-full/${ARCH}/${BRANCH}"
-  "org.freedesktop.Sdk.Extension.llvm17/${ARCH}/${BRANCH}"
-  "org.freedesktop.appstream-glib/${ARCH}/stable"
-)
-
-retry_command sudo apt-get -qq update
-
-# Install packages needed for building
-echo "Will install the following packages for building - ${BUILD_PACKAGES[*]}"
-retry_command sudo apt-get -y install "${BUILD_PACKAGES[@]}"
-
-sudo flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
-
-# Install packages needed for building
-echo "Will install the following packages for building - ${FLATPAK_PACKAGES[*]}"
-retry_command sudo flatpak -y install "${FLATPAK_PACKAGES[@]}"
-
-echo "Installing Flatpak Builder"
-retry_command sudo flatpak -y install flathub org.flatpak.builder
-
-echo "Downloading flat-manager-client"
-mkdir -p "$FLAT_MANAGER_CLIENT_DIR"
-pushd "$FLAT_MANAGER_CLIENT_DIR"
-aria2c -Z "https://raw.githubusercontent.com/flatpak/flat-manager/9401efbdc0d6bd489507d8401c567ba219d735d5/flat-manager-client"
-chmod +x flat-manager-client
-echo "$FLAT_MANAGER_CLIENT_DIR" >> $GITHUB_PATH
-popd