Yarn provides a yarnpkg/setup-action action you can use in your CI workflows:

name: JavaScript

on:
  push:

jobs:
  typecheck:
    name: Typecheck
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # 6.0.2

      - name: Setup Yarn
        uses: yarnpkg/setup-action@c9778d0717d1b46cf29f06ae5ae5ad41c1bb1c1a

      - name: Running TypeScript
        run: |
          yarn tsc