selenium-server-standalone: fix useChrome = false

Previously when useChrome was set to false the command would be something like

```
makeWrapper [...] \
  --add-flags "-cp [...]" \
  --add-flags  \
  --add-flags "org.openqa.grid.selenium.GridLauncherV3"
```

The problem was that only the arguments to `--add-flags` was optional while for this to be parsed correctly the flag and argument has to be optional.

pkgs/development/tools/selenium/server/default.nix

@@ -26,7 +26,7 @@ in stdenv.mkDerivation rec {
     cp $src $out/share/lib/${pname}-${version}/${pname}-${version}.jar
     makeWrapper ${jre}/bin/java $out/bin/selenium-server \
       --add-flags "-cp $out/share/lib/${pname}-${version}/${pname}-${version}.jar:${htmlunit-driver}/share/lib/${htmlunit-driver.name}/${htmlunit-driver.name}.jar" \
-      --add-flags ${optionalString chromeSupport "-Dwebdriver.chrome.driver=${chromedriver}/bin/chromedriver"} \
+      ${optionalString chromeSupport "--add-flags -Dwebdriver.chrome.driver=${chromedriver}/bin/chromedriver"} \
       --add-flags "org.openqa.grid.selenium.GridLauncherV3"
   '';