Update cache mechanism for R packages

[zkamvar]

Our cache system is in the workflows is not working and that's because I did not have a good model of how the caching worked previously. Take the following code:

styles/.github/workflows/website.yml

Lines 55 to 79 in da77111

	- name: Restore R Cache
	if: steps.check-rmd.outputs.count != 0
	uses: actions/cache@v2
	with:
	path: ${{ env.R_LIBS_USER }}
	key: ${{ runner.os }}-${{ hashFiles('.github/R-version') }}-1-${{ hashFiles('.github/depends.Rds') }}
	restore-keys: ${{ runner.os }}-${{ hashFiles('.github/R-version') }}-1-
	- name: Install needed packages
	if: steps.check-rmd.outputs.count != 0
	run: |
	source('bin/dependencies.R')
	install_required_packages()
	shell: Rscript {0}
	- name: Query dependencies
	if: steps.check-rmd.outputs.count != 0
	run: |
	source('bin/dependencies.R')
	deps <- identify_dependencies()
	create_description(deps)
	use_bioc_repos()
	saveRDS(remotes::dev_package_deps(dependencies = TRUE), ".github/depends.Rds", version = 2)
	writeLines(sprintf("R-%i.%i", getRversion()$major, getRversion()$minor), ".github/R-version")
	shell: Rscript {0}

Line 60 depends on the files created by Lines 77 and 78, respectively. What needs to happen is that, of these three steps, "Restore R Cache" should go at the end. I wrote a bit about this the last time I had to wrestle this issue: carpentries/actions#13 (comment)