Sandbox the OUT_DIR environment variable

Currently, OUT_DIR is inherited from the parent process, leading to
scripts being able to find the output directory when the enviornment
variable is set to an absolute path. When sandboxing a command,
also rewrite the OUT_DIR environment variable to the sandboxed one,
so that scripts can't find the real out dir.

Bug: 307824623
Test: Presubmits
Change-Id: I325071121a60bddc4105df680fbdfe3d11dc94e2