Factor out Machine::systemSupported

There's just enough logic (the `"builtin"` special case) that makes this
worthy of its own method.

src/build-remote/build-remote.cc

@@ -137,9 +137,8 @@ static int main_build_remote(int argc, char * * argv)
                 for (auto & m : machines) {
                     debug("considering building on remote machine '%s'", m.storeUri);
 
-                    if (m.enabled
-                        && (neededSystem == "builtin"
-                            || m.systemTypes.count(neededSystem) > 0) &&
+                    if (m.enabled &&
+                        m.systemSupported(neededSystem) &&
                         m.allSupported(requiredFeatures) &&
                         m.mandatoryMet(requiredFeatures))
                     {

src/libstore/machines.cc

@@ -38,6 +38,11 @@ Machine::Machine(decltype(storeUri) storeUri,
     sshPublicHostKey(sshPublicHostKey)
 {}
 
+bool Machine::systemSupported(const std::string & system) const
+{
+    return system == "builtin" || (systemTypes.count(system) > 0);
+}
+
 bool Machine::allSupported(const std::set<std::string> & features) const
 {
     return std::all_of(features.begin(), features.end(),

src/libstore/machines.hh

@@ -19,6 +19,12 @@ struct Machine {
     const std::string sshPublicHostKey;
     bool enabled = true;
 
+    /**
+     * @return Whether `system` is either `"builtin"` or in
+     * `systemTypes`.
+     */
+    bool systemSupported(const std::string & system) const;
+
     /**
      * @return Whether `features` is a subset of the union of `supportedFeatures` and
      * `mandatoryFeatures`