CI: Disable Valgrind test for now

This test has consistently failed for months as it takes too long.

While it should be looked into its not helpful to show CI as always failing either.

.github/workflows/check.yml

@@ -216,6 +216,7 @@ jobs:
     name: Run tests in valgrind
     needs: check # Don't run expensive test if main check fails
     runs-on: ubuntu-20.04 # Might as well test with a different one too
+    if: ${{ false }} # Currently Valgrind takes too long and always fails
     steps:
     - name: Install Dependencies
       run: |