StemaOS Devkit Client - Missing rsync

**Game information**
SteamOS Devkit Client

**Flatpak info**
[flatpak --version]
Flatpak 1.16.1

[flatpak --gl-drivers]
nvidia-580-126-09
default
host

**Problem description**
SteamOS Devkit Client does not work because the rsync tool is not available inside the flatpak.
The application requires rsync to work and it would be nice to have it available.

I was able to expose my system rsync to the application with `flatpak-spawn` as a test and it progressed further but pairing with a dev-kit failed because system rsync can't see necessary files.

**Does this issue reproduce with native Steam**
Have not tested (probably not though given the problem)

I don't know much about flatpaks but would it be possible to make rsync available?

Start by locating the Flatpak packaging metadata for com.valvesoftware.Steam and checking how runtime tools are declared. Reproduce the SteamOS Devkit Client launch and pairing flow, then verify that rsync is available inside the sandbox and pairing succeeds; the issue provides no named file or test.

Written by the indexing model from the issue text.