Reduce stack usage in ckati

When limiting ckati to 768kB of stack, it would crash with a segfault
when processing this recursive expanded variable. So always use := to
turn this into a simple variable.

I measured this using 772kB of stack in ckati on sdk_x86_64-eng. With
this change, the next highest stack user is 106kB.

Bug: 36182021
Test: ulimit -s 768; lunch sdk_x86_64-eng; m nothing
Test: build-sdk_x86_64.ninja is identical before/after this change
Change-Id: I20a6396db173decf178107e3aac6cf089e541e36