chore[ci]: downgrade codecov action to v3 (vyperlang#3940)

there is a bug in the action since february which is resulting in
intermittently failing builds. this issue introduced in 730679b,
which upgraded the codecov action.

references:
GH codecov/codecov-action#1280

.github/workflows/test.yml

@@ -159,7 +159,7 @@ jobs:
           tests/
 
     - name: Upload Coverage
-      uses: codecov/codecov-action@v4
+      uses: codecov/codecov-action@v3
       with:
         token: ${{ secrets.CODECOV_TOKEN }}
         file: ./coverage.xml
@@ -220,7 +220,7 @@ jobs:
           tests/
 
     - name: Upload Coverage
-      uses: codecov/codecov-action@v4
+      uses: codecov/codecov-action@v3
       with:
         token: ${{ secrets.CODECOV_TOKEN }}
         file: ./coverage.xml