The docker image is based bitbake-builder. 'sudo su - builder' step added for
oe-selftest as 'USER builder' doesn't really load user level profile. The below
vars are misssing before user re-switch.

  SHELL=/usr/bin/bash
  LOGNAME=builder
  LESSCLOSE=/usr/bin/lesspipe %s %s
  LESSOPEN=| /usr/bin/lesspipe %s
  USER=builder
  MAIL=/var/mail/builder

And PATHes are different before and after user re-switch.
Before: PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
After: PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin

Although 'whoami' command can be found in both PATHes, oe-selftest reports
'whoami' not found issue with the former path, the reason behind is to be
investigated. In general, user instead of root profile should be used for
oe-selftest.

Signed-off-by: Chase Qi <chase.qi@linaro.org>