Squeak: Specify exact Trunk image version

[LinqLover]

Trying to realize the idea I mentioned here: #471 (comment)

Usage: smalltalkci -s "Squeak{{ 32 | 64}}-{{ version_number }}, where version_number points to a Squeak version such as 19431.

Work in progress, let's see whether it works ... :-)

[LinqLover]

Only one failure left which seems unrelated to my changes. Apart from that, the MCMcmUpdater>>#doUpdate:upToUpdate: proposal is pending discussion on squeak-dev: The Inbox: MonticelloConfigurations-ct.166

[fniephaus]

What happens if I request a trunk image with update level less than the one of the base image that smalltalkCI uses? We'll probably have to update trunk base images some time in the future again, so this is quite likely. I don't think we can downgrade images, or can we?

[LinqLover]

Afaik downgrading is not possible. My current approach always uses the latest image that was released before the requested version number, see squeak::latest_release_before() (https://github.com/hpi-swa/smalltalkCI/pull/506/files#diff-f15a119f2269fdd82da32c9e83444165027e5d69e93ecc94bed8da5f86761ade) :-)