Phase 6: DRY `tag_version` derivation in `release.yml`

[thomaschristory]

tag_version="${GITHUB_REF_NAME#v}" is re-derived in three separate run: blocks in .github/workflows/release.yml (HEAD = f323c78):

• Step "Verify tag matches package version" (line ~63)
• Step "Verify wheel filename matches tag" (line ~80)
• Step "Create GitHub Release" (line ~150)

Fix

Set the variable once via $GITHUB_ENV in the first verify step:

echo "TAG_VERSION=${GITHUB_REF_NAME#v}" >> "$GITHUB_ENV"

Then reference $TAG_VERSION in the subsequent steps.

Why deferred from v1

Pure cosmetic; the duplication is correct, just not DRY. Workflow runs and passes. Filed for tracking; would be merged opportunistically when next touching release.yml.