Reland "[lldb][lit] Add MallocNanoZone envvar to Darwin ASan builds" … (#88442)

…(#88436)"

This reverts commit 1f5d130d. The
original commit checks that the host system is "Darwin" before setting
the `MallocNanoZone` envvar, but on the Shell lit config this attribute
does not exist at the point where it is being checked which leads to a
build failure.

This commit checks the host OS correctly.