[MINOR][R][BUILD] More reliable detection of R version for Windows in AppVeyor

## What changes were proposed in this pull request?

This PR proposes to use https://rversions.r-pkg.org/r-release-win instead of https://rversions.r-pkg.org/r-release to check R's version for Windows correctly.

We met a syncing problem with Windows release (see #15709) before. To cut this short, it was ...

- 3.3.2 release was released but not for Windows for few hours.
- `https://rversions.r-pkg.org/r-release` returns the latest as 3.3.2 and the download link for 3.3.1 becomes `windows/base/old` by our script
- 3.3.2 release for WIndows yet
- 3.3.1 is still not in `windows/base/old` but `windows/base` as the latest
- Failed to download with `windows/base/old` link and builds were broken

I believe this problem is not only what we met. Please see https://github.com/krlmlr/r-appveyor/commit/01ce943929993bbf27facd2cdc20ae2e03808eb4 and also this `r-release-win` API came out between 3.3.1 and 3.3.2 (assuming to deal with this issue), please see `https://github.com/metacran/rversions.app/issues/2`.

Using this API will prevent the problem although it looks quite rare assuming from the commit logs in https://github.com/metacran/rversions.app/commits/master. After 3.3.2, both  `r-release-win` and `r-release` are being updated together.

## How was this patch tested?

AppVeyor tests.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18859 from HyukjinKwon/use-reliable-link.