ci: pin uv, the tool that builds the pytest venv

The panel backend has 25 pytest tests that no workflow has ever run. Making
them run needs a throwaway virtualenv, and uv is what builds it in seconds
against the pinned requirements-dev.txt.

Installing it through install-ci-tools.sh rather than assuming it is on the
runner keeps the version in one place, where the other three tools already
live, and where the Renovate regex manager can move it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/tool-versions.env

@@ -7,3 +7,5 @@
 ACTIONLINT_VERSION="1.7.7"
 SHELLCHECK_VERSION="0.11.0"
 KUBECONFORM_VERSION="0.8.0"
+# uv builds the throwaway venv the pytest job runs in.
+UV_VERSION="0.12.17"