[ci] add CMake + R 3.6 test back (fixes #3469)

.ci/test_r_package_windows.ps1

@@ -28,6 +28,24 @@ function Run-R-Code-Redirect-Stderr {
   Rscript --vanilla -e $decorated_code
 }
 
+# Remove all items matching some pattern from PATH environment variable
+function Remove-From-Path {
+  param(
+    [string]$item_to_remove
+  )
+  $env:path = ($env:path.Split(';') | Where-Object { $_ -notmatch "$item_to_remove" }) -join ';'
+}
+
+# remove some details that exist in the GitHub Actions images which might
+# cause conflicts with R and other components installed by this script
+$env:RTOOLS40_HOME = ""
+Remove-From-Path ".*chocolatey.*"
+Remove-From-Path ".*Chocolatey.*"
+Remove-From-Path ".*Git.*mingw64.*"
+Remove-From-Path ".*msys64.*"
+Remove-From-Path ".*rtools40.*"
+Remove-From-Path ".*Strawberry.*"
+
 # Get details needed for installing R components
 #
 # NOTES:
@@ -95,6 +113,10 @@ Write-Output "Installing Rtools"
 ./Rtools.exe /VERYSILENT /SUPPRESSMSGBOXES /DIR=$RTOOLS_INSTALL_PATH ; Check-Output $?
 Write-Output "Done installing Rtools"
 
+# wait for all Rtools files to be written
+Write-Output "Sleeping to allow Rtools install to finish"
+Start-Sleep -Seconds 60
+
 Write-Output "Installing dependencies"
 $packages = "c('data.table', 'jsonlite', 'Matrix', 'processx', 'R6', 'testthat'), dependencies = c('Imports', 'Depends', 'LinkingTo')"
 Run-R-Code-Redirect-Stderr "options(install.packages.check.source = 'no'); install.packages($packages, repos = '$env:CRAN_MIRROR', type = 'binary', lib = '$env:R_LIB_PATH')" ; Check-Output $?

.github/workflows/r_package.yml

@@ -60,6 +60,12 @@ jobs:
             compiler: clang
             r_version: 4.0
             build_type: cmake
+          - os: windows-latest
+            task: r-package
+            compiler: MINGW
+            toolchain: MINGW
+            r_version: 3.6
+            build_type: cmake
           - os: windows-latest
             task: r-package
             compiler: MINGW