support: fix mk-venv with relative paths

This patch fixes a bug in mk-venv where it would misconfigure the
environment, if started with a relative pathname.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/fix-mk-venv/ by tarmac [r=zkrynicki][bug=][author=zkrynicki]"