local-derivation-goal.cc: downgrade "warning: rewriting hashes in..." down to debug

Before the changes when building the whole system with
`contentAddressedByDefault = true;` we get many noninformative messages:

    $ nix build -f nixos system --keep-going
    ...
    warning: rewriting hashes in '/nix/store/...-clang-11.1.0.drv.chroot/nix/store/...-11.1.0'; cross fingers
    warning: rewriting hashes in '/nix/store/...-clang-11.1.0.drv.chroot/nix/store/...-11.1.0-dev'; cross fingers
    warning: rewriting hashes in '/nix/store/...-clang-11.1.0.drv.chroot/nix/store/...-11.1.0-python'; cross fingers
    error: 2 dependencies of derivation '/nix/store/...-hub-2.14.2.drv' failed to build
    warning: rewriting hashes in '/nix/store/...-subversion-1.14.1.drv.chroot/nix/store/...-subversion-1.14.1-dev'; cross fingers
    warning: rewriting hashes in '/nix/store/...-subversion-1.14.1.drv.chroot/nix/store/...-subversion-1.14.1-man'; cross fingers
    ...

Let's downgrade these messages down to debug().

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index e91e35851..b1852a6bb 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -2208,7 +2208,7 @@ void LocalDerivationGoal::registerOutputs()
         auto rewriteOutput = [&]() {
             /* Apply hash rewriting if necessary. */
             if (!outputRewrites.empty()) {
-                warn("rewriting hashes in '%1%'; cross fingers", actualPath);
+                debug("rewriting hashes in '%1%'; cross fingers", actualPath);
 
                 /* FIXME: this is in-memory. */
                 StringSink sink;