Specify the release python version in only place

[llimeht]

Description

In reviewing #3652, @krzywon wondered if we could abstract to just one place where we specified the default python version for flows and for embedding in the installer.

This PR does so, by putting the version into a variable within ci.yml, and then when generating the test/build matrix in matrix.py, using the current interpreter as the release interpreter, thus bringing that config with it.

(This PR also fixes a stray 3.12 that crept in via a different parallel PR to the one moving to 3.13.)

How Has This Been Tested?

CI to check that the right versions are used where intended. It's 3.13 everywhere except in the places where we want 3.12 for testing, just as desired.

Review Checklist:

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Documentation (check at least one)

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)