build.sh: carry FP6IMG_NO_CROSSDIRECT across the su re-exec

su scrubs the environment, so the escape hatch from the previous commit
never reached the build user (verified: libcamera still built crossdirect
and failed identically).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

build.sh

@@ -72,7 +72,8 @@ if [ "$(id -u)" = 0 ]; then
 	# would poison the next podman invocation's cwd)
 	install -d -o build dist
 	find dist -mindepth 1 -delete
-	exec su build -c "sh -eu '$PWD/build.sh'"
+	# su scrubs the environment — carry the knobs that matter across it
+	exec su build -c "FP6IMG_NO_CROSSDIRECT='${FP6IMG_NO_CROSSDIRECT:-}' sh -eu '$PWD/build.sh'"
 fi
 
 # git hosts occasionally hiccup (our forgejo lives on a small VM that also