Create StringSet DerivationBuilderParams::systemFeatures

Do this to avoid checking "system features" from the store config
directly, because we rather not have `DerivationBuilder` depend on
`Store`.

src/libstore/build/derivation-building-goal.cc

@@ -766,6 +766,7 @@ Goal::Co DerivationBuildingGoal::tryToBuild()
                     .initialOutputs = initialOutputs,
                     .buildMode = buildMode,
                     .defaultPathsInChroot = std::move(defaultPathsInChroot),
+                    .systemFeatures = worker.store.config.systemFeatures.get(),
                     .finalEnv = std::move(finalEnv),
                     .extraFiles = std::move(extraFiles),
                 });

src/libstore/include/nix/store/build/derivation-builder.hh

@@ -65,6 +65,14 @@ struct DerivationBuilderParams
      */
     PathsInChroot defaultPathsInChroot;
 
+    /**
+     * May be used to control various platform-specific functionality.
+     *
+     * For example, on Linux, the `kvm` system feature controls whether
+     * `/dev/kvm` should be exposed to the builder within the sandbox.
+     */
+    StringSet systemFeatures;
+
     struct EnvEntry
     {
         /**

src/libstore/unix/build/linux-derivation-builder.cc

@@ -492,7 +492,7 @@ struct ChrootLinuxDerivationBuilder : ChrootDerivationBuilder, LinuxDerivationBu
             createDirs(chrootRootDir + "/dev/shm");
             createDirs(chrootRootDir + "/dev/pts");
             ss.push_back("/dev/full");
-            if (store.Store::config.systemFeatures.get().count("kvm") && pathExists("/dev/kvm"))
+            if (systemFeatures.count("kvm") && pathExists("/dev/kvm"))
                 ss.push_back("/dev/kvm");
             ss.push_back("/dev/null");
             ss.push_back("/dev/random");