wrap: Set the env var to adjust libshared-library-guard configuration path

When inside a Flatpak container, we expect that libshared-library-guard.so looks for its blocklist configuration in /app/etc/freedesktop-sdk.ld.so.blockedlist.

This will not work because we bind /app in /run/parent/app. For this reason, when we detect this situation, we set the environment variable SHARED_LIBRARY_GUARD_CONFIG to point libshared-library-guard in the correct path.

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This requires https://github.com/flathub/com.valvesoftware.Steam/pull/754 and more importantly https://gitlab.com/freedesktop-sdk/shared-library-guard/-/merge_requests/23

If the upstream patches don't get merged, as an alternative, we could bind in the sub-sandbox libshared-library-guard.so and freedesktop-sdk.ld.so.blockedlist in the same /app directory

/cc @smcv