Have you tried running it as-is? I believe tinytex is supposed to handle installing missing dependencies on its own.
If it doesn't, you can probably just add another step after setting up tinytex to manually install packages, such as:
steps:
- uses: actions/checkout@v2
- uses: r-lib/actions/setup-tinytex@master
- uses: r-lib/actions/setup-r@v1
- name: add TeX deps
run: |
tinytex::tlmgr_install(pkgs = "name")
shell: Rscript {0}