I am in the process of setting it up on a cheap Hetzner box. If it works, would be a great deal! You can get a 64 GB RAM box for 35 EUR/mo at server auctions with unlimited traffic. I don't mention CPU or GPU, as typically this isn't a bottleneck for my projects.
Plus, I can configure cache sharing via host-mounted dir. E.g. pnpm cache can be all in one place, and be locally available to pods via a mounted dir. Same for the Docker image cache. This would speed up CI runs and also reduce network traffic by a huge margin.
GitHub Actions effectively has no local caching. There's an action for caching, but it uses a blob storage for cache artifacts. Which then gets network fetched, gzip'ed and gunzip'ed each time, and from my experience this has never been a gain for medium to large npm projects, as they have thousands of small .js files in node_modules, and thus takes a long time to compress and decompress. I think npm edge cache servers are already so optimized and fast, that in my experience almost always it's faster to install from npm directly. I even tested this on AWS, where the cache was stored in S3, in the same region as CodeBuild (CI), and direct installs from npm were still faster by about 30%.
So other than adding more hardware resources, local caching is the only way to significantly speed up GH Actions, from my experience, and thus you must have your runner.