[llvm] [GitHub] Remove redundant cache key prefix (PR #76914)

[Tobias Hieta via llvm-commits]

tru wrote:

On a side note, I think we should look into how many cache hits we get in a build and see if we can improve it, I don't think we have good enough hits since the builds still takes quite a while. In our internal jobs we run `ccache -s` after each build to see how many hits and misses we have, maybe we can do something similar with sscache? Just writing it here to see if someone wants to pick that task up :)

https://github.com/llvm/llvm-project/pull/76914