Minor bug: sccache --show-stats randomizes order of non-cachable reasons

[SeseMueller]

Similar to #386 and #397.

When running sccache --show-stats and two of the non-cachable reasons have the same number of occurences, their order is random.
As with #386, this is likely because the entries are printed in the order the HashMap.iter() returns them.

This only ever has the effect that running the command multiple times can have slightly different outputs even if nothing changed.

Minor bug, low priority.