[scripts] fix TERM handling

Patch 01d639cf changed boot arg handling
by saving them to a map rather than reading them out of the environment,
with the effect that now later boot args override earlier ones.

Given this new behavior, this CL moves the default TERM handling to the
beginning of the boot args so that the caller can override it.

Test: `scripts/run-zircon-x64 -Cb -c "TERM=dumb"` then `echo $TERM`
      now properly shows "dumb"

Change-Id: Id4f78bb159a65a16eaa64172154ca15372cb20b7