Prior to this commit, pkgbase-stage.lua used a hard-coded PKG_DBDIR
"./pkgdb"; unfortunately this creates a race condition if we start
building disc1.iso and dvd1.iso at the same time

pkg: sqlite error while executing CREATE TABLE licenses [...]
in file pkgdb.c:2330: table licenses already exists

since pkg checks to see if the pkgdb is initialized and initializes
it if not.

Use separate pkgdb-disc1 and pkgdb-dvd directories.

MFC after: 1 minute
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