[ci] fix R CMD check issue 'unable to verify current time' (#3341)

.ci/test_r_package.sh

@@ -7,6 +7,10 @@ mkdir -p $R_LIB_PATH
 export R_LIBS=$R_LIB_PATH
 export PATH="$R_LIB_PATH/R/bin:$PATH"
 
+# hack to get around this:
+# https://stat.ethz.ch/pipermail/r-package-devel/2020q3/005930.html
+export _R_CHECK_SYSTEM_CLOCK_=0
+
 # Get details needed for installing R components
 #
 # NOTES:

.ci/test_r_package_windows.ps1

@@ -74,6 +74,10 @@ $env:CTAN_MIRROR = "https://ctan.math.illinois.edu/systems/win32/miktex"
 $env:CTAN_MIKTEX_ARCHIVE = "$env:CTAN_MIRROR/setup/windows-x64/"
 $env:CTAN_PACKAGE_ARCHIVE = "$env:CTAN_MIRROR/tm/packages/"
 
+# hack to get around this:
+# https://stat.ethz.ch/pipermail/r-package-devel/2020q3/005930.html
+$env:_R_CHECK_SYSTEM_CLOCK_ = 0
+
 if (($env:COMPILER -eq "MINGW") -and ($env:R_BUILD_TYPE -eq "cmake")) {
   $env:CXX = "$env:RTOOLS_MINGW_BIN/g++.exe"
   $env:CC = "$env:RTOOLS_MINGW_BIN/gcc.exe"