[ci] remove pin on tinytex in R CI jobs

[jameslamb]

Reverts part of #5807.

Windows R jobs are failing on master with the same error as reported in #5802.

I suspect the root cause is that the changes from r-lib/actions#712 have now made it into v2 of the r-lib/setup-tinytex action.

[jameslamb]

I don't totally understand, but it seems like CRAN (both from RStudio and the official CRAN) have stopped serving {lattice} to R 3.6 🤷🏻

official CRAN

docker run \
    --rm \
    rocker/verse:3.6 \
    Rscript -e "install.packages('lattice', repos = 'https://cran.r-project.org')"

Installing package into ‘/usr/local/lib/R/site-library’
(as ‘lib’ is unspecified)
Warning message:
package ‘lattice’ is not available (for R version 3.6.0)

RStudio

docker run \
    --rm \
    rocker/verse:3.6 \
    Rscript -e "install.packages('lattice', repos = 'https://cran.rstudio.com')"

Installing package into ‘/usr/local/lib/R/site-library’
(as ‘lib’ is unspecified)
Warning message:
package ‘lattice’ is not available (for R version 3.6.0)

MRAN

docker run \
    --rm \
    rocker/verse:3.6 \
    Rscript -e "install.packages('lattice', repos = 'https://mran.microsoft.com')"

Installing package into ‘/usr/local/lib/R/site-library’
(as ‘lib’ is unspecified)
trying URL 'https://mran.microsoft.com/src/contrib/lattice_0.20-45.tar.gz'
Content type 'application/octet-stream' length 399470 bytes (390 KB)
==================================================
downloaded 390 KB

* installing *source* package ‘lattice’ ...
...
* DONE (lattice)

[github-actions]

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