Remove sccache from Rust install #3728
                
     Merged
            
            
          
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resolves #3696
While removing sccache is somewhat of a blunt hammer, rustc incremental builds have gotten pretty good such that it can be more complication than it's worth. Once #2885 lands (it's ready; just needs some testing, so hopefully this week) we can update the nightly version or move to stable, if possible, to benefit from better incremental builds.