fix issue where signing a message sometimes doesn't allow you to scroll to the bottom (#15788)

* fix issue wherein signing a message sometimes doesn't allow you to scroll all the way to the bottom

done in a way to make it as mergeable as possible

* Fixing length-zero-no-unit lint issue

Co-authored-by: ryanml <ryanlanese@gmail.com>

ui/components/app/signature-request/signature-request-footer/index.scss

@@ -1,6 +1,6 @@
 .signature-request-footer {
   display: flex;
-  border-top: 1px solid var(--color-border-muted);
+  box-shadow: inset 0 1px 0 var(--color-border-muted);
 
   button {
     text-transform: uppercase;