Install to a subdirectory of ~/.steam for new installations

As discussed on #916303, conflating $STEAMCONFIG with $STEAMDIR breaks intended functionality, because $STEAMCONFIG/steam and $STEAMDIR/steam are intended to have different purposes, and cannot both work as intended if they collide.

Closes: #916303
Signed-off-by: Simon McVittie smcv@collabora.com

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I've checked that this does the right thing in a user account that previously had the ~/.local/share/Steam setup (it continues to be used), a user account that previously had the ~/.steam setup that conflates $STEAMCONFIG and $STEAMDIR (it continues to be used), and a new user account (we install with STEAMDIR = ~/.steam/debian-installation).