slapgrid: run promise with clean environment

This fixes bug https://project.nexedi.net/#/bug_module/20210603-1C66F4F
12 jobs for fix/promise_clean_environment in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

02:12:46

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:51

passed SlapOS.Eggs.UnitTest-Master.Python3

00:17:55

failed SlapOS-Master.UnitTest-Master

01:51:36

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:14

passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:53

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:00

passed SlapOS.Eggs.UnitTest-Master.Python2

00:15:03

passed SlapOS.Eggs.UnitTest-Master.Python3

00:21:08

passed SlapOS.Eggs.UnitTest-Master.Python3

00:17:18

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:00

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:22