When updating Steam build on linux, game fails to run due to libraries

[darloth]

When updating the game from a steam install on linux (Ubuntu 20.04.4 LTS) following the process:

1. Install on steam
2. Point launchpad at steam install location
3. select "Update Game" button on Manage OpenSR tab

The game will then not launch due to steam having an outdated version of libcurl in the lin64 folder. This does not occur when installing OpenSR2 to a fresh empty directory.

[DaloLorn]

• Detect upgrade from commercial/non-OSR build. Sort-of done already, I think, but I'll need to check.
• When upgrading, rename old bin folder to bin_legacy.
• Add new button (and associated launcher params!) to run the legacy binaries, intended primarily for releasing updates to the Steam Workshop.