Upgrade to codecov uploader v2

Some time ago codecov stopped supporting their old v1 uploader, and
it seems they've now finally turned it off, so we aren't getting
any coverage reports anymore. Hopefully upgrading is pretty trivial.

Author: TheBlueMatt

.github/workflows/build.yml

@@ -172,7 +172,7 @@ jobs:
           done
       - name: Upload coverage
         if: matrix.coverage
-        uses: codecov/codecov-action@v1
+        uses: codecov/codecov-action@v2
         with:
           # Could you use this to fake the coverage report for your PR? Sure.
           # Will anyone be impressed by your amazing coverage? No