[ci] changed strategy for installing R on AppVeyor

[jameslamb]

@StrikerRUS while working on microsoft/LightGBM#2936, I noticed that in this repo you're using a link like this to get R.exe:

appveyor DownloadFile https://cloud.r-project.org/bin/windows/base/R-${R_VERSION}-win.exe

I think that those links only work as long as that version is the latest release. In this PR, I propose changing to the /old URLs which you can rely on forever (so you can upgrade at your own pace).

[StrikerRUS]

@jameslamb Wow, thank you very much! I have already encountered this problem twice. I thought about using some loop with possible future version numbers but wasn't smart enough to look for the permanent link 🙂

Only one note.
I think we can define R_VERSION: 3.6.2 in R-package/.R.appveyor.ps1 file to be consistent with Travis configuration:

rgf/R-package/.R.travis.sh

Lines 16 to 17 in ba62e1f

	if ! command -v R &> /dev/null; then
	R_VER=3.6.2

[jameslamb]

@jameslamb Wow, thank you very much! I have already encountered this problem twice. I thought about using some loop with possible future version numbers but wasn't smart enough to look for the permanent link 🙂

Only one note.
I think we can define R_VERSION: 3.6.2 in R-package/.R.appveyor.ps1 file to be consistent with Travis configuration:

rgf/R-package/.R.travis.sh

Lines 16 to 17 in ba62e1f

	if ! command -v R &> /dev/null; then
	R_VER=3.6.2

sure! changed it in a3bac27

[StrikerRUS]

Thank you @jameslamb !