[llvm] [Github] Default to non-root user in linux CI container (PR #119987)

[Tom Stellard via llvm-commits]

https://github.com/tstellar approved this pull request.

This is fine with me.  The only downside is that we then can't install new packages in the container from a Workflow.  We could fix this by giving the user sudo acess, though.

https://github.com/llvm/llvm-project/pull/119987