Make public keys and requireSigs local-store specific again

Thanks @regnat and @edolstra for catching this and comming up with the
solution.

They way I had generalized those is wrong, because local settings for
non-local stores is confusing default. And due to the nature of C++
inheritance, fixing the defaults is more annoying than it should be.
Additionally, I thought we might just drop the check in the substitution
logic since `Store::addToStore` is now streaming, but @regnat rightfully
pointed out that as it downloads dependencies first, that would still be
too late, and also waste effort on possibly unneeded/unwanted
dependencies.

The simple and correct thing to do is just make a store method for the
boolean logic, keeping all the setting and key stuff the way it was
before. That new method is both used by `LocalStore::addToStore` and the
substitution goal check. Perhaps we might eventually make it fancier,
e.g. sending the ValidPathInfo to remote stores for them to validate,
but this is good enough for now.

src/libstore/local-store.hh

@@ -35,6 +35,10 @@ struct LocalStoreConfig : virtual LocalFSStoreConfig
 {
     using LocalFSStoreConfig::LocalFSStoreConfig;
 
+    Setting<bool> requireSigs{(StoreConfig*) this,
+        settings.requireSigs,
+        "require-sigs", "whether store paths should have a trusted signature on import"};
+
     const std::string name() override { return "Local Store"; }
 };
 
@@ -71,6 +75,8 @@ private:
            minFree but not much below availAfterGC, then there is no
            point in starting a new GC. */
         uint64_t availAfterGC = std::numeric_limits<uint64_t>::max();
+
+        std::unique_ptr<PublicKeys> publicKeys;
     };
 
     Sync<State> _state;
@@ -88,6 +94,12 @@ public:
     const Path tempRootsDir;
     const Path fnTempRoots;
 
+private:
+
+    const PublicKeys & getPublicKeys();
+
+public:
+
     // Hack for build-remote.cc.
     PathSet locksHeld;
 
@@ -124,6 +136,8 @@ public:
     void querySubstitutablePathInfos(const StorePathCAMap & paths,
         SubstitutablePathInfos & infos) override;
 
+    bool pathInfoIsTrusted(const ValidPathInfo &) override;
+
     void addToStore(const ValidPathInfo & info, Source & source,
         RepairFlag repair, CheckSigsFlag checkSigs) override;