Stash changes before checkout on check size CI job

We need to do this because package-json may differ after installing
on the CI machine. We should find a better way to deal with this in the
future.

Author: brizental

bin/build-size-check.sh

@@ -16,6 +16,9 @@ npm run build:webext
 webext_size=$(wc -c ./dist/glean.js | awk '{print $1}')
 webext_size_pretty=$(wc -c ./dist/glean.js | awk '{printf "%0.2f\n",$1/1024"."substr($2,1,2)}')
 
+# TODO: we are doing this because of package-lock changes,
+# but we should really find a better way to deal with such issues.
+git stash
 git checkout -b original-main origin/main
 
 npm install