Tweaks #249

	name: CI
	on:
	- push
	jobs:
	ci:
	name: Lint and build
	runs-on: ubuntu-latest
	steps:
	- uses: actions/checkout@v2
	with:
	# `PrairieLearn` is a submodule
	submodules: true

	# Yarn cache
	- name: Get yarn cache directory path
	id: yarn-cache-dir-path
	run: echo "::set-output name=dir::$(yarn cache dir)"

	- uses: actions/cache@v2
	with:
	path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
	key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
	restore-keys: \|
	${{ runner.os }}-yarn-

	# Next.js build cache
	- uses: actions/cache@v2
	with:
	path: ${{ github.workspace }}/.next/cache
	key: ${{ runner.os }}-nextjs-${{ hashFiles('**/yarn.lock') }}

	- name: Install
	run: yarn install --frozen-lockfile

	- name: Lint
	run: yarn lint

	- name: Build
	run: yarn build

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