ci: Port Citra flatpak build to yuzu

Port Citra's travis based flatpak build
to yuzu's azure build system.

.ci/scripts/linux-flatpak/finish.sh

@@ -0,0 +1,10 @@
+#!/bin/bash -ex
+
+YUZU_SRC_DIR="/yuzu"
+REPO_DIR="$YUZU_SRC_DIR/repo"
+
+# When the script finishes, unmount the repository and delete sensitive files,
+# regardless of whether the build passes or fails
+umount "$REPO_DIR" || true
+rm -rf "$REPO_DIR" "/tmp/*" || true
+