build-remote: Fix fallback to other machines when connecting fails

Opening an SSHStore or LegacySSHStore does not actually establish a
connection, so the try/catch block here did nothing. Added a
Store::connect() method to test whether a connection can be
established.

src/build-remote/build-remote.cc

@@ -167,6 +167,7 @@ int main (int argc, char * * argv)
                         storeParams["ssh-key"] = bestMachine->sshKey;
 
                     sshStore = openStore(bestMachine->storeUri, storeParams);
+                    sshStore->connect();
                     storeUri = bestMachine->storeUri;
 
                 } catch (std::exception & e) {