slapgrid: fix invocation of bootstrapBuildout
This commit reverts ddd77222. The goal is to fix nexedi/slapos.buildout!25 (comment 148861) os.environ is a special mapping object that transparently modifies the actual environment and we should never make os.environ make point to something else (see nexedi/slapos.buildout!25 (comment 149333) and https://docs.python.org/3/library/os.html#os.environ) Instead change the environment only when calling SlapPopen. We will see if the problems fixed by ddd77222 appear again...