[1868WY] fix available variants on trains owned by corporations

[michaeljb]

Fixes #10783

• the base code determined that both the 4 and 4+3 variants should be available for other corporations to buy (other code made sure the UI only presented the 4+3, which is how the train was bought initially by LNP)
• FE&MV has $325, more than the face value of a 4-train ($300) but less than the 4+3's face value ($360)
• the 1868 Wyoming code assumes that if two variants are available, we're buying from the depot
• when buying from the depot, we can only buy a train we can afford or the cheapest, so the 4+3 gets filtered out

The fix is to simply return the single active variant if the train is in fact owned by a corporation instead of the depot.

Before clicking "Create"

• Branch is derived from the latest master
• Add the pins or archive_alpha_games label if this change will break existing games
• Code passes linter with docker compose exec rack rubocop -a
• Tests pass cleanly with docker compose exec rack rake