Pinned dependency files not used during release build process

[krzywon]

Describe the bug
When PRs against a branch named release* is triggered, the OS-specific dependency lists, that are pinned, are supposed to be used, but they are not.

See the build for 3980. Under Install correct versions..., bumps v1.0.4 is installed, but the dependency files have v1.0.5rc0 pinned in them suggesting the unpinned requirements.txt file is used.

Expected behavior
The requirements-{{ $os}}.txt files should be used instead.

SasView version (please complete the following information):

• Version: 6.2.0a3

Operating system (please complete the following information):

• OS: All

Installation method (please complete the following information):

• Installed: Windows INstaller

[llimeht]

I think the issue is that the pinning has been done in the wrong place - it needs to be at the top of the github action where the branch is selected for the sibling projects.

Note that pip can't cache wheels built if the pinning is done against a git checkout / git branch / git tag.