fix issues with non-writable temp directories

[lachlan-roberts]

some cleanups for temp directory creation

fixes the following error when deploying with EE10

java.lang.IllegalArgumentException: Temp dir /workspace not useable: writeable=false, dir=true
    at org.eclipse.jetty.server.handler.ContextHandler.createTempDirectory(ContextHandler.java:702)
    at org.eclipse.jetty.ee10.webapp.WebAppContext.createTempDirectory(WebAppContext.java:472)
    at org.eclipse.jetty.ee10.webapp.WebInfConfiguration.preConfigure(WebInfConfiguration.java:63)
    at org.eclipse.jetty.ee10.webapp.Configurations.preConfigure(Configurations.java:487)
    at org.eclipse.jetty.ee10.webapp.WebAppContext.preConfigure(WebAppContext.java:454)
    at org.eclipse.jetty.ee10.webapp.WebAppContext.doStart(WebAppContext.java:495)
    at com.google.apphosting.runtime.jetty.ee10.AppEngineWebAppContext.doStart(AppEngineWebAppContext.java:212)