Add `cache` action?

[ulysses4ever]

I'm very illiterate in caching of haskell builds in GHA, so take with a grain of salt. Not small part of it is ugly and bulky incantations people usually use there. E.g cabal's workflow:

      - if: ${{ runner.os != 'Windows' }}
        uses: actions/cache@v3
        with:
          # validate.sh uses a special build dir
          path: |
            ${{ steps.setup-haskell.outputs.cabal-store }}
            dist-*
          key: ${{ runner.os }}-${{ matrix.ghc }}-20220419-${{ github.sha }}
          restore-keys: ${{ runner.os }}-${{ matrix.ghc }}-20220419-

Julia (the language) had very nice cache action that makes the task a one liner and hides complexity. Could we have the same? https://github.com/julia-actions/cache

[andreasabel]

Indeed bulky, see my recent recipe:

actions/setup/README.md

Lines 154 to 174 in f102749

	- name: Restore cached dependencies
	uses: actions/cache/restore@v3
	id: cache
	with:
	path: ${{ steps.setup.outputs.cabal-store }}
	key: ${{ runner.os }}-ghc-${{ env.GHC_VERSION }}-cabal-${{ env.CABAL_VERSION }}-plan-${{ hashFiles('**/plan.json') }}
	restore-keys: |
	${{ runner.os }}-ghc-${{ env.GHC_VERSION }}-cabal-${{ env.CABAL_VERSION }}-
	- name: Install dependencies
	run: cabal build all --only-dependencies
	# Cache dependencies already here, so that we do not have to rebuild them should the subsequent steps fail.
	- name: Save cached dependencies
	uses: actions/cache/save@v3
	# Caches are immutable, trying to save with the same key would error.
	if: ${{ !steps.cache.outputs.cache-hit
	|| steps.cache.outputs.cache-primary-key != steps.cache.outputs.cache-matched-key }}
	with:
	path: ${{ steps.setup.outputs.cabal-store }}
	key: ${{ steps.cache.outputs.cache-primary-key }}

There is an old PR #11 which could maybe be revived:

• Add 'cache: true' option to enable caching #11

I think a caching option should enable the user to state whether dist-newstyle should be cached or not, as it can be too big (like 0.5G for Agda).