Install LaTeX in CI to enable testing PDF export format

`nbconvert` requires a LaTeX engine to convert to `pdf`. We need to figure out the best way to install this in our CI pipeline so we can test the `pdf` export format in our unit tests.

Some strategies and notes about them:
- [ ] Install standard TeX distributions per operating system (TeX Live, MacTeX, MikTeX).
- Will require different conditional pipeline steps, which adds complexity to the pipeline. Example of install scripts from LightGBM: [ubuntu](https://github.com/microsoft/LightGBM/blob/e83042f20633d7f74dda0d18624721447a610c8b/.ci/test_r_package.sh#L43-L52), [macos](https://github.com/microsoft/LightGBM/blob/e83042f20633d7f74dda0d18624721447a610c8b/.ci/test_r_package.sh#L57-L61), windows [1](https://github.com/microsoft/LightGBM/blob/e83042f20633d7f74dda0d18624721447a610c8b/.ci/test_r_package_windows.ps1#L15-L30), [2](https://github.com/microsoft/LightGBM/blob/e83042f20633d7f74dda0d18624721447a610c8b/.ci/test_r_package_windows.ps1#L35-L38), [3](https://github.com/microsoft/LightGBM/blob/e83042f20633d7f74dda0d18624721447a610c8b/.ci/test_r_package_windows.ps1#L99-L108)
- [ ] Install [`texlive-core`](https://github.com/conda-forge/texlive-core-feedstock) from conda-forge.
- [X] Out of box: does not work. `nbconvert` uses `xelatex` but this installation only has `pdflatex`.
- [ ] Hack it by symlinking `xelatex` to `pdflatex`
- [ ] Set `nbconvert` configuration to use `pdflatex` instead of `xelatex`
- [ ] Install [`tectonic`](https://github.com/conda-forge/tectonic-feedstock), a different TeX distribution that's available cross-platform from conda-forge
- [X] Out of box: does not work. This gives a CLI program named `tectonic`
- [X] Hack it by symlinking `xelatex` to `tectonic`: Does not work. `nbconvert` passes a `--quiet` flag that `tectonic` does not support.
- [ ] Set `nbconvert` configuration to use `tectonic` instead of `xelatex` (and with appropriate CLI flags)
- [ ] Install [`tinytex`](https://yihui.org/tinytex/). Has install scripts for different OSes. Will need different conditional pipeline steps, which adds complexity to the pipeline, but scripts hopefully work out of the box.