installer: populate installer::globals::[os,com,cpuname]

with there respective env variable OS,COM,CPUNAME
this is a first step to get rid of OUTPATH

Change-Id: Idab4dd7b19deba5252390cbd2cb74dfff9efe64e
Reviewed-on: https://gerrit.libreoffice.org/11907
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>