ci: fix hardcoded artifact paths in workflows

This change fixes hardcoded artifact paths by using ${{ env.VARDIR }}
expression.

At this moment VARDIR=/tmp/tnt, so affected workflows are:
- debug_aarch64.yml
- memtx_allocator_based_on_malloc.yml
- osx_11_aarch64.yml
- osx_11_aarch64_debug.yml

NO_DOC=ci
NO_CHANGELOG=ci
NO_TEST=ci