abuild: also pass through ABUILD_BOOTSTRAP in rootbld

since 1582617eb8, it's not passed,
so it's not possible to skip tests for a run via a
 $ ABUILD_BOOTSTRAP=1 abuild rootbld

abuild.in

@@ -2504,6 +2504,7 @@ rootbld() {
 		--chdir "$startdir" \
 		--clearenv \
 		--setenv HOME "$HOME" \
+		${ABUILD_BOOTSTRAP:+--setenv ABUILD_BOOTSTRAP "$ABUILD_BOOTSTRAP"} \
 		--setenv SOURCE_DATE_EPOCH "$SOURCE_DATE_EPOCH" \
 		--setenv ABUILD_LAST_COMMIT "$ABUILD_LAST_COMMIT" \
 		--setenv PATH ${USE_CCACHE:+/usr/lib/ccache/bin:}/bin:/usr/bin:/sbin:/usr/sbin \