Store::registerDrvOutput make pure virtual

It should be the responsibility of implementations that don't implement
it to say so.

See also PR #9799, and issue #5729

src/libstore/dummy-store.cc

@@ -258,6 +258,11 @@ struct DummyStore : virtual Store
         return path;
     }
 
+    void registerDrvOutput(const Realisation & output) override
+    {
+        unsupported("registerDrvOutput");
+    }
+
     void narFromPath(const StorePath & path, Sink & sink) override
     {
         bool visited = contents.cvisit(path, [&](const auto & kv) {

src/libstore/include/nix/store/legacy-ssh-store.hh

@@ -109,7 +109,7 @@ struct LegacySSHStore : public virtual Store
         unsupported("addToStore");
     }
 
-    virtual StorePath addToStoreFromDump(
+    StorePath addToStoreFromDump(
         Source & dump,
         std::string_view name,
         FileSerialisationMethod dumpMethod = FileSerialisationMethod::NixArchive,
@@ -121,6 +121,11 @@ struct LegacySSHStore : public virtual Store
         unsupported("addToStore");
     }
 
+    void registerDrvOutput(const Realisation & output) override
+    {
+        unsupported("registerDrvOutput");
+    }
+
 public:
 
     BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv, BuildMode buildMode) override;

src/libstore/include/nix/store/store-api.hh

@@ -598,10 +598,7 @@ public:
      * floating-ca derivations and their dependencies as there's no way to
      * retrieve this information otherwise.
      */
-    virtual void registerDrvOutput(const Realisation & output)
-    {
-        unsupported("registerDrvOutput");
-    }
+    virtual void registerDrvOutput(const Realisation & output) = 0;
 
     virtual void registerDrvOutput(const Realisation & output, CheckSigsFlag checkSigs)
     {