Use Chromium binary from system for capturing screenshots

This commit makes screenshot tests work on macOS running on Apple silicon,
but adds Chromium as an external dependency for running them.

See puppeteer/puppeteer#6622 for more details.

runScreenshotTests

@@ -50,6 +50,8 @@ do
     test_details="Target path: $(magenta $target_path) | "
     test_details+="Viewport width: $(magenta $viewport_width)"
     echo -en "[$(gray ....)] ${test_details} "
+    PUPPETEER_EXECUTABLE_PATH=`which chromium` \
+    PUPPETEER_SKIP_CHROMIUM_DOWNLOAD=true \
     ./comparePages $target_path $viewport_width && {
       echo -e "\r[$(bold_green PASS)] ${test_details}"
     } || {