Update Github Actions workflows for cache step to v4.

[fontikar]

From 1/Feb Github Actions will no longer support v1-2 of actions/cache

We need to update to v3/v4

My suggestion is to update to v4 and see if it breaks anything
Looking at the changelog here, it doesn't look like updating to v3 or v4 will introduce any breaking actions to this workflow

The lines to change:

• smatr/.github/workflows/R-CMD-check.yaml

  Line 54 in fcc3f68

  uses: actions/cache@v2

• smatr/.github/workflows/check-coverage.yml

  Line 33 in fcc3f68

  uses: actions/cache@v2

• smatr/.github/workflows/pkgdown.yml

  Line 29 in fcc3f68

  uses: actions/cache@v2

• smatr/.github/workflows/lint.yml

  Line 31 in fcc3f68

  uses: actions/cache@v2