Harden version detection

[keynmol]

We're using this java file:

public class PrintJavaVersion {
  public static void main(String[] args) {
    System.out.print(System.getProperty("java.version"));
  }
}

compiled with java 8 and converted to base64, so that we can reliably print java version regardless whether we run JDK or JRE.

Test plan

• Existing tests pass

[keynmol]

This is probably unnecessarily defensive, but disappearing temp files are not unheard of.