Update Compatibility Report Template to reference log location for Flatpak based Steam installs

Problem:

Currently the Compatibility Report Template does not include the filepath for the generated .log if Steam was installed via Flatpak:

<!-- Please add `PROTON_LOG=1 %command%` to the game's launch options and
attach the generated $HOME/steam-$APPID.log to this issue report as a file.
(Proton logs compress well if needed.)-->

Solution:

Update the instructions to reference Flatpak filepath (~/.var/app/com.valvesoftware.Steam/) so something like:

<!-- Please add `PROTON_LOG=1 %command%` to the game's launch options and
attach the generated steam-$APPID.log which will be found as $HOME/steam-$APPID.log or if you installed Steam as a Flatpak,  ~/.var/app/com.valvesoftware.Steam/steam-$APPID.log, to this issue report as a file.
(Proton logs compress well if needed.)-->

The issue identifies the Compatibility Report Template and provides the current and proposed instruction text. Locate that template, review the existing Proton log guidance, and update it to cover both standard and Flatpak Steam log locations. Verify the resulting wording preserves the launch-option and attachment instructions.