Use 5.9.0 explicitly to avoid getting 5.9.1

[czechboy0]

Motivation

In Swift 5.9.1, there were some improvements to corelibs-foundation around Sendability, however we need to keep supporting 5.9.0 toolchains.

One thing I learned today is that if you specify "5.9" from the docker registry, you might get "5.9.1", which we explicitly don't want, as we could miss issues that only show up on "5.9.0".

One option would be to have two pipelines, one for 5.9.0 and 5.9.1, however I don't think it's necessary, as we already have a 5.10 pipeline, verifying that newer toolchains work.

This PR bumps down the 5.9 pipeline explicitly to use 5.9.0.

Modifications

Explicitly require 5.9.0, instead of using the latest 5.9 toolchain.

Result

More issues are caught in CI instead of only hitting users who use 5.9.0 later.

Test Plan

Ran locally, still passes.

[simonjbeaumont]

Closed as superseded by #40