tests: don't read /etc/abuild.conf

Host's /etc/abuild.conf may enable USE_CCACHE which will pull in ccaches
as makedepends which breaks the tests.

fixes https://gitlab.alpinelinux.org/alpine/abuild/-/issues/10017

tests/abuild.bats

@@ -1,6 +1,7 @@
 setup() {
 	export ABUILD="$PWD/../abuild"
 	export ABUILD_SHAREDIR="$PWD/.."
+	export ABUILD_CONF=/dev/null
 	export REPODEST="$BATS_TMPDIR"/packages
 }