Cache timing appears to be broken

[luser]

I noticed this while looking at the log in #1756. All of the cache hit log lines read Cache hit in 0.000 s. It looks like this got broken at some point. The duration is calculated here:

sccache/src/compiler/compiler.rs

Line 268 in 62c8f0a

let duration = start.elapsed();

but that's prior to where it actually awaits the cache get future:

sccache/src/compiler/compiler.rs

Line 277 in 62c8f0a

let lookup = match cache_status.await {

This also means the summary stats don't work either:

Average cache read hit            0.000 s