Reduce workarounds

• _v2-entry-point: Rely on pv-wrap reading environment variables itself

  This requires pressure-vessel!79.

• _start-container-in-background: Rely on pv-wrap --pass-fd

  Now that we have those options, we don't need this additional layer of shell.

  This requires pressure-vessel!79.

• depot: Don't set STEAM_RUNTIME=/ for the container

  This assumes pressure-vessel sets it when appropriate (pressure-vessel!81).

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Also includes !15 (closed).

Marked as WIP because a suitable pressure-vessel has not yet been released.

/cc @denittis