call twine by absolute path; PATH does not cross a step boundary

The zig fix worked: both manylinux_2_17 wheels and the sdist built. Then the
Publish step died with 'twine: not found', because the PATH the build step
exported for zig's sake belongs to that step's shell and not the next one, and
the same commit had changed twine to a bare name on the strength of it.

Only the two commands in the build step, where the export actually applies,
are bare now.

.forgejo/workflows/release.yml

@@ -70,8 +70,10 @@ jobs:
           TWINE_USERNAME: __token__
           TWINE_PASSWORD: ${{ secrets.PYPI_TOKEN }}
         run: |
-          twine check --strict dist/*
+          # By absolute path: each step gets its own shell, so the PATH the
+          # build step exported for zig's sake does not reach this one.
+          /tmp/venv/bin/twine check --strict dist/*
           # --skip-existing so a retry after a partial upload finishes the job
           # instead of dying on the first file. A version cannot be reused,
           # so a half-published release would otherwise burn it.
-          twine upload --non-interactive --skip-existing dist/*
+          /tmp/venv/bin/twine upload --non-interactive --skip-existing dist/*