dashboard: fix the workaround for unpacking node sources

dashboard: fix the workaround for unpacking node sources

My previous workaround in the dashboard for the unpacking of non-root
own tarball as the fake root of a container did not work because of the
strange quoting/escaping behavior of cmake (it tried to run `id -u` as a
single command, not a command and an argument).
Use single quoted string and old school backticks to work around this issue.

Fixes: 24dbfb5da4813c6588f9cd199b9f527bb67f1e88
Signed-off-by: John Mulligan <jmulligan@redhat.com>