Fix generation of environment variable list

The command below in src/start.sh appends an "=" to each environment variable name in the generated list:

$ env | grep -v -w '_' | awk -F: '{ st = index($0,"=");print substr($1,0,st) ","}' | tr -d "\n"
SHELL=,SESSION_MANAGER=,QT_ACCESSIBILITY=,COLORTERM=, ...

Changing the argument to awk -F removes the "=", as required by su -w

$ env | grep -v -w '_' | awk -F= '{ st = index($0,"=");print substr($1,0,st) ","}' | tr -d "\n"
SHELL,SESSION_MANAGER,QT_ACCESSIBILITY,COLORTERM, ...

Partial fix for #90

balena-io-experimental / browser

Fix generation of environment variable list #91