Fix remaining tslint errors (YAY).

- Also updated ci.yml to ensure that linting failures will break the build.

Fully fixes https://b.corp.google.com/issues/411384603

packages/cli/src/ui/components/Header.tsx

@@ -7,7 +7,7 @@ interface HeaderProps {
   cwd: string;
 }
 
-const Header: React.FC<HeaderProps> = ({ cwd }) => (
+export const Header: React.FC<HeaderProps> = ({ cwd }) => (
   <>
     {/* Static Header Art */}
     <Box marginBottom={1}>
@@ -34,5 +34,3 @@ const Header: React.FC<HeaderProps> = ({ cwd }) => (
     </Box>
   </>
 );
-
-export default Header;