Unit "build wall" commands are always queued

[zoot]

If a gathering citizen soldier is given a command (without holding down Shift) to build a wall, it will not execute it immediately, but add it to its command queue.

Reproduced on Acropolis 1 playing Gauls:

1. Select a citizen soldier and command it to gather.
2. While it is gathering, command it to build a Palisade.
3. The unit continues to gather instead executing the command immediately.

[Deiz]

In 12676:

Wall build orders shouldn't always be queued. Fixes #1673.