Add zkg wrapper which does not pull down sources.

When executing `zkg` concurrently multiple updates to the sources can
race and destroy each others state. Disable that update. This is a
workaround for zeek/package-manager#139.

tests/scripts/zkg

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+# This script provides a wrapper around `zkg` which provides a workaround for
+# zeek/package-manager#139. The approach taken here is to provide an empty
+# `sources` section in the zkg configuration; this seems to disable updates of
+# the default package source.
+
+STATE=$(realpath "$(mktemp -d zkg.state.XXXXXX)")
+CONFIG=$(mktemp config.XXXXXX)
+
+trap 'rm -rf ${STATE} ${CONFIG}' EXIT
+
+cat > "$CONFIG" << _END_
+[templates]
+default = https://github.com/zeek/package-template
+
+[paths]
+state_dir = ${STATE}/.zkg
+script_dir = ${STATE}/.zkg/script_dir
+plugin_dir = ${STATE}/.zkg/plugin_dir
+bin_dir = ${STATE}/.zkg/bin
+zeek_dist =
+
+[sources]
+_END_
+
+zkg --configfile "$CONFIG" "$@"