Linux: Fix PRIME detection on Steam
Linux: Fix PRIME detection on Steam

To avoid trying to do PRIME detection on fake `libGL.so` as used by e.g.
Renderdoc or Primus, we skip detection if there's a `libGL.so` in
`LD_LIBRARY_PATH`... and our luck is that Steam defines it and includes
system paths too, thus the actual system `libGL`... 🤦

So if we detect Steam, we skip this check.

Co-authored-by: Hein-Pieter van Braam-Stewart <e68b072303e1c28c4073630daeb803737a761e06@tmm.cx>

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