cabal freeze file is not so suitable as cache key

[andreasabel]

Thanks for publishing this action! Here a comment on cache keys:

haskell-example/.github/workflows/build.yml

Lines 63 to 75 in ed8660f

	- name: Freeze
	run: >
	cabal freeze
	--builddir=${{ env.cabal-build-dir }}
	- name: Cache
	uses: actions/cache@v3
	env:
	hash: ${{ hashFiles('cabal.project.freeze') }}
	with:
	key: ${{ matrix.os }}-ghc-${{ matrix.ghc }}-${{ env.hash }}
	restore-keys: |
	${{ matrix.os }}-ghc-${{ matrix.ghc }}-

Since cabal.project.freeze will contain the Hackage repository timestamp, it will be different in almost every single CI run. Thus, it might not be a suitable cache key, being similar to ${{ github.sha }}.

I settled for dist-newstyle/cache/plan.json instead which only contains the package versions, no time stamps. It is generated by cabal build --dry-run.

See e.g. https://github.com/agda/agda/blob/e018e58453a06e4c4a15c9b81396cbcd1e2c640f/src/github/workflows/cabal.yml#L148-L171

There might be reasons why you want the primary key being different in each run, but then it is easier to just use ${{ github.sha }}.