Make sure correct version of SCT is installed in the release automation workflow

[joshuacwnewton]

Checklist

GitHub

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• I've consulted SCT's internal developer documentation to ensure my contribution is in line with any relevant design decisions
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Description

In #3383, I added a GitHub Actions workflow to automate the manual steps for creating a release. I had tested it on a separate repo, and it seemed to work OK.

However, I realize I made a mistake: In the GitHub action, I installed SCT from the release branch. This will generate the wrong requirements-freeze.txt; what I actually needed to do was:

1. Install SCT from the master branch
2. After the correct, up-to-date dependencies have been installed in the conda environment, then checkout the release branch and do the remaining steps (merge master, update requirements-freeze.txt and version.txt, etc.)

I've tested this once more in https://github.com/joshuacwnewton/test-github-actions/actions/runs/1271642941 and I feel confident that this will properly create a release in SCT.

Linked issues

N/A.

[kousu]

Alright seems good!

Can we get rid of release eventually do you think?