pbulk: Avoid cleaning $prefix on every bulk build in master mode.

Whatever is in $prefix shouldn't affect the result -- both scanning
and building happen in the workers -- and it is a bit of a nasty
surprise for merely issuing bulkbuild-restart to nuke all the host's
installed packages.

ok joerg

pkgtools/pbulk/files/pbulk/scripts/pre-build

@@ -1,5 +1,5 @@
 #!@SH@
-# $NetBSD: pre-build,v 1.15 2020/01/08 15:12:01 joerg Exp $
+# $NetBSD: pre-build,v 1.16 2021/01/02 21:54:46 riastradh Exp $
 #
 # Copyright (c) 2007 Joerg Sonnenberger <[email protected]>.
 # All rights reserved.
@@ -84,7 +84,22 @@ fi
 
 mkdir -p "${bulklog}" "${loc}"
 
-@PREFIX@/libexec/pbulk/client-clean
+case "${master_mode}" in
+[nN][oO])
+	# Make sure the prefix is clean before we scan so any
+	# preinstalled packages don't affect the scanning process.
+	@PREFIX@/libexec/pbulk/client-clean
+	;;
+[yY][eE][sS])
+	# Scanning happens in a worker so we don't need to clean the
+	# prefix here; this way you can run pbulk on a system that has
+	# packages installed without disrupting those packages.
+	;;
+*)
+	echo "master_mode must be either yes or no."
+	exit 1
+	;;
+esac
 
 # Log common settings...
 opsys=`cd ${pkgsrc}/pkgtools/pkg_install && ${make} show-var VARNAME=OPSYS`