Unset BUILD_NUMBER/HOSTNAME in kati.go

BUILD_NUMBER/HOSTNAME is passed by a file, not env variable.
To avoid kati recreation, unset BUILD_NUMBER/HOSTNAME after
writing it into a file

Bug: 278060169
Test: BUILD_NUMBER=1 m && BUILD_NUMBER=2 m (check there is no kati
re-run)

Change-Id: I5a31461dcf1e4b0634974bcb48a0d7482d42852a