xvnc in jenkins stack - can not create /home/bitnami/.vnc

I am using the ami-005cd330 image with jenkins 1.475 together with xvnc jenkins plugin (http://wiki.hudson-ci.org/display/HUDSON/Xvnc+Plugin). The job is configured to start xvnc prior to build. The log of the job is as follows:

    ...
    Starting xvnc
    [prjx] $ vncserver :28
    vncserver: Could not create /home/bitnami/.vnc.
    Starting xvnc
    [prjx] $ vncserver :29
    vncserver: Could not create /home/bitnami/.vnc.
    FATAL: Failed to run 'vncserver :29' (exit code 13), blacklisting display #29; consider checking the     "Clean up before start" option
    java.io.IOException: Failed to run 'vncserver :29' (exit code 13), blacklisting display #29;  consider checking the "Clean up before start" option
    at hudson.plugins.xvnc.Xvnc.doSetUp(Xvnc.java:100)
    ...
    

I thought that jenkins executes the build commands as bitnami user, but this seems not to be true. When I output the environment variables using "env" USER and USERNAME are root, but HOME is set to /home/bitnami. What can I do so that jenkins (or better xvnc) is able to start?