make mutable borrowing more explicit during cache writing
make mutable borrowing more explicit during cache writing

It's not clear to me whether something like this is already happening
inside type derivation or not, but this at least makes things clearer
that `compiler_result` is not being borrowed mutably.

glandium/sccache,glandium/sccache,glandium/sccache,mozilla/sccache,mozilla/sccache,luser/sccache2,mozilla/sccache,glandium/sccache,luser/sccache2,luser/sccache2,luser/sccache2