binary-cache-public-keys -> trusted-public-keys

The name had become a misnomer since it's not only for substitution
from binary caches, but when adding/copying any
(non-content-addressed) path to a store.

src/libstore/crypto.cc

@@ -105,7 +105,7 @@ PublicKeys getDefaultPublicKeys()
 
     // FIXME: filter duplicates
 
-    for (auto s : settings.binaryCachePublicKeys.get()) {
+    for (auto s : settings.trustedPublicKeys.get()) {
         PublicKey key(s);
         publicKeys.emplace(key.name, key);
     }