[llvm] workflows/release-binaries: Replace some workflow interpolations with env vars (PR #120860)

[Tom Stellard via llvm-commits]

tstellar wrote:

> Seems reasonable enough to me.
> 
> Is there a plan to update other workflows that we have?

I might do it if I have extra time, but I wanted to update this file, because I'm making modifications to it in another PR that adds more of these.

https://github.com/llvm/llvm-project/pull/120860