[CI] fix R's installation

[guolinke]

No description provided.

[jameslamb]

@guolinke what was the cause for this?

[guolinke]

@jameslamb I don't know, the master branch builds suddenly failed.

[jameslamb]

ok no problem. I'm worried that this is going to result in us getting a different R version than the one we want, but if it fixes a problem it's ok to leave it for a while and see how it goes. I'll keep an eye on the logs.

[guolinke]

@jameslamb you can refer to https://github.com/microsoft/LightGBM/runs/953772345#step:4:375

[guolinke]

BTW, seems some R's tests are stuck forever.

[jameslamb]

ugh, weird!

It does look like it's working ok on master.

On r-package (ubuntu-latest, gcc, R 3.6, cmake) (https://github.com/microsoft/LightGBM/runs/953865741)

dpkg: warning: downgrading r-base-dev from 4.0.2-1.1804.0 to 3.6.3-1bionic
Preparing to unpack .../11-r-base-dev_3.6.3-1bionic_all.deb ...
Unpacking r-base-dev (3.6.3-1bionic) over (4.0.2-1.1804.0) ...

and on r-package (r-package (ubuntu-latest, gcc, R 4, cmake)) (https://github.com/microsoft/LightGBM/runs/953865780):

r-base-dev is already the newest version (4.0.2-1.1804.0).
r-base-dev set to manually installed.

[github-actions]

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