[ci] [R-package] R 3.6 Linux CI jobs broken, cannot install {Matrix}

[jameslamb]

Description

R 3.6 Linux CI jobs are failing, with the following relevant logs.

Warning: unable to access index for repository https://mran.microsoft.com/src/contrib:
  cannot open URL 'https://mran.microsoft.com/src/contrib/PACKAGES'
Warning message:
package ‘lattice’ is not available (for R version 3.6.3) 
Warning: dependency ‘lattice’ is not available
...
ERROR: dependency ‘lattice’ is not available for package ‘Matrix’
...
Warning message:
In install.packages(c("data.table", "jsonlite", "knitr", "Matrix",  :
  installation of one or more packages failed,
  probably ‘Matrix’
...
ERROR: dependency ‘Matrix’ is not available for package ‘lightgbm’
      -----------------------------------
ERROR: package installation failed

Reproducible example

Both R 3.6 Linux CI jobs, across multiple PRs (example build link).

Environment info

N/A

Additional Comments

The macOS R3.6 jobs were removed from this project recently for the same issue: #5859.

I'm not sure what changed on MRAN or with R recently to cause these failures.

[github-actions]

This issue has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.