Address CI errors with R 4.6.0

[khaeru]

R version 4.6.0 was released 2026-04-24.

With this version, the "ubuntu-latest tutorials" and "windows-latest tutorials" CI jobs fail, e.g. here, with messages like those below.

To work around, the R version has been pinned to <4.6 in the workflow file in #1010.

To resolve:

• Find or file a bug upstream.
• Wait for a fix or recommended mitigation/change.
• Apply.

On ubuntu-latest:

Error in dyn.load(file, DLLpath = DLLpath, ...) : 
  unable to load shared object '/home/runner/work/_temp/Library/rlang/libs/rlang.so':
ERROR: lazy loading failed for package ‘dplyr’
* removing ‘/home/runner/work/_temp/Library/dplyr’
* restoring previous ‘/home/runner/work/_temp/Library/dplyr’
  /home/runner/work/_temp/Library/rlang/libs/rlang.so: undefined symbol: SETLENGTH
Calls: <Anonymous> ... namespaceImport -> loadNamespace -> library.dynam -> dyn.load
Execution halted

On windows-latest:

Error in inDL(x, as.logical(local), as.logical(now), ...) : 
  unable to load shared object 'D:/a/_temp/Library/base64enc/libs/x64/base64enc.dll':
  LoadLibrary failure:  The specified procedure could not be found.
Calls: loadNamespace ... asNamespace -> loadNamespace -> library.dynam -> dyn.load -> inDL
Execution halted