Avoid fallback download of artifacts when not on develop

[maurelian]

Summary

Avoids the fallback download of artifacts when not on develop.

This is achieved by refactoring the get-target-branch logic from CircleCI config into a reusable script that can be sourced by the use-latest-fallback script

Changes

• Created scripts/ops/get-target-branch.sh containing the target branch detection logic
• Updated scripts/ops/use-latest-fallback.sh to source the new script
• Updated scripts/checks/check-semver-diff.sh to source the new script
• Updated .circleci/config.yml to use the new script

Benefits

• Scripts are now self-contained and determine TARGET_BRANCH themselves
• Can be tested locally without CircleCI environment variables
• Single source of truth for target branch logic
• Less duplication in CircleCI config

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.3%. Comparing base (aa2b4d3) to head (46de590).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #18788     +/-   ##
==========================================
- Coverage     76.3%    76.3%   -0.1%     
==========================================
  Files          560      560             
  Lines        53363    53363             
==========================================
- Hits         40739    40733      -6     
- Misses       12480    12486      +6     
  Partials       144      144             

Flag	Coverage Δ
cannon-go-tests-64	66.4% <ø> (ø)
contracts-bedrock-tests	77.6% <ø> (ø)
unit	77.0% <ø> (-0.1%)	⬇️

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see 4 files with indirect coverage changes

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