Run coverage via Github Action

Note: I tried directly invoking `codecov`, but it "couldn't tell" that
it was running in a GA/CI container, so I've opted for using the GA
codecov integration, which does work as expected.

.github/workflows/validate.yml

@@ -15,3 +15,7 @@ jobs:
       with:
         node-version: ${{ matrix.node-version }}
     - run: make validate.ci
+    - name: Upload coverage
+      uses: codecov/codecov-action@v1
+      with:
+        fail_ci_if_error: true

Makefile

@@ -66,4 +66,3 @@ validate:
 validate.ci:
 	npm ci
 	make validate
-	codecov