Improve CI build performance by utilizing uv caching

[nijel]

Describe the problem

The CI build could be optimized if built wheels could be reused because building of wheels like borgbackup or mercurial is what takes most of the time there.

Describe the solution you would like

• use https://github.com/reproducible-containers/buildkit-cache-dance to make caching work across CI builds
• invoke uv with RUN --mount type=cache as descibed in https://github.com/moby/buildkit/blob/dockerfile/1.4.3/frontend/dockerfile/docs/syntax.md#run---mounttypecache
• uv cache prune --ci as described in https://docs.astral.sh/uv/guides/integration/github/#caching

Describe alternatives you have considered

No response

Screenshots

No response

Additional context

No response

[github-actions]

This issue seems to be a good fit for newbie contributors. You are welcome to contribute to Weblate! Don't hesitate to ask any questions you would have while implementing this.

You can learn about how to get started in our contributors documentation.