chore: disable unused-but-set-variable for clang (#1441)

At some point (I believe version 13.0.0?) clang added a warning that
catches variables that were assigned but unused. This version of clang
(or later) is now bundled w/ github's macos images and is causing our
tests to fail in continuous integration. We can currently generate C
code that trips this warning, so for now I've disabled it as we do some
other warnings related to variable usages.

app/Main.hs

@@ -36,6 +36,7 @@ defaultProject =
             "-Wall",
             "-Werror",
             "-Wno-unused-variable",
+            "-Wno-unused-but-set-variable",
             "-Wno-self-assign"
           ],
       projectLibFlags = case platform of