Proof VF if there is one, static if not

Makefile

@@ -41,7 +41,7 @@ test: venv-test build.stamp
 	TOCHECK=$$(find fonts/variable -type f 2>/dev/null); if [ -z "$$TOCHECK" ]; then TOCHECK=$$(find fonts/ttf -type f 2>/dev/null); fi ; . venv-test/bin/activate; mkdir -p out/ out/fontbakery; fontbakery check-googlefonts -l WARN --full-lists --succinct --badges out/badges --html out/fontbakery/fontbakery-report.html --ghmarkdown out/fontbakery/fontbakery-report.md $$TOCHECK  || echo '::warning file=sources/config.yaml,title=Fontbakery failures::The fontbakery QA check reported errors in your font. Please check the generated report.'
 
 proof: venv build.stamp
-	. venv/bin/activate; mkdir -p out/ out/proof; diffenator2 proof $(shell find fonts/ttf -type f) -o out/proof
+	TOCHECK=$$(find fonts/variable -type f 2>/dev/null); if [ -z "$$TOCHECK" ]; then TOCHECK=$$(find fonts/ttf -type f 2>/dev/null); fi ; . venv/bin/activate; mkdir -p out/ out/proof; diffenator2 proof $$TOCHECK -o out/proof
 
 images: venv $(DRAWBOT_OUTPUT)