Refactor mw sqlite related env variables

Move SQLITE_DIR from mw-install-sqlite.sh to mw-set-env.sh and rename it
to MW_DB_PATH (matching $wgDbPath).
Move DB_NAME as well and rename it  MW_DB_NAME.

That will let us easily tear down the sqlite database file after job
completion, thus reclaiming disk space.

Bug: 71128
Change-Id: I7b0a4aedd7cacae74db386d946a28e27f42e89a6