Move genflags tool to cmd/ and pin to previous release

to alleviate problems caused by the circular dependency of using the
same code as a library that is potentially being generated to adhere to
a different API.

.github/workflows/cli.yml

@@ -43,6 +43,9 @@ jobs:
       - name: test
         run: go run internal/build/build.go test
 
+      - name: test urfave-cli-genflags
+        run: make -C cmd/urfave-cli-genflags
+
       - name: check-binary-size
         run: go run internal/build/build.go check-binary-size