ci: Remove Python 3.8 fallback to pip install

[matthewfeickert]

Description

• Use uv for all installations in CI, as the Python 3.8 fall back to pip install was needed only for TensorFlow.
  • Amends PR feat: Remove support for TensorFlow #2609

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Use uv for all installations in CI, as the Python 3.8 fall back to
  'pip install' was needed only for TensorFlow.
   - Amends PR https://github.com/scikit-hep/pyhf/pull/2609

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (10488f0) to head (b69ff91).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2619   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files          65       65           
  Lines        4193     4193           
  Branches      591      591           
=======================================
  Hits         4119     4119           
  Misses         45       45           
  Partials       29       29           

Flag	Coverage Δ
contrib	97.97% <ø> (ø)
doctest	98.09% <ø> (ø)
unittests-3.10	96.27% <ø> (ø)
unittests-3.11	96.27% <ø> (ø)
unittests-3.12	96.27% <ø> (ø)
unittests-3.8	96.27% <ø> (ø)
unittests-3.9	96.32% <ø> (ø)

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[matthewfeickert]

I'm going to approve and merge this myself. As always, PRs approved by a single core dev can be reverted as needed by the rest of the dev team.