[ci] stop relying on cran.microsoft.com in CI jobs (microsoft#5909)

.ci/test_r_package.sh

@@ -120,12 +120,8 @@ fi
 
 # fix for issue where CRAN was not returning {lattice} when using R 3.6
 # "Warning: dependency ‘lattice’ is not available"
-#
-# refs for that MRAN snapshot:
-# * https://cran.r-project.org/web/packages/checkpoint/readme/README.html
-# * https://help.codeocean.com/en/articles/3087704-using-mran-snapshots-to-install-archived-r-packages
 if [[ "${R_MAJOR_VERSION}" == "3" ]]; then
-    Rscript --vanilla -e "install.packages('lattice', repos = 'https://cran.microsoft.com/snapshot/2020-04-23/', lib = '${R_LIB_PATH}')"
+    Rscript --vanilla -e "install.packages('https://cran.r-project.org/src/contrib/Archive/lattice/lattice_0.20-41.tar.gz', repos = NULL, lib = '${R_LIB_PATH}')"
 fi
 
 # Manually install Depends and Imports libraries + 'knitr', 'RhpcBLASctl', 'rmarkdown', 'testthat'