Set _R_CHECK_SYSTEM_CLOCK_ to false to skip the system clock check …

…in `devtools::check_built` (mlflow#4542)

* set _R_CHECK_SYSTEM_CLOCK_ to false

Signed-off-by: harupy <17039389+harupy@users.noreply.github.com>

* Clarify comment

Signed-off-by: harupy <17039389+harupy@users.noreply.github.com>

* Fix comment and uppercase false

Signed-off-by: harupy <17039389+harupy@users.noreply.github.com>

.github/workflows/master.yml

@@ -98,6 +98,14 @@ jobs:
         cd tests
         Rscript ../.create-test-env.R
     - name: Run tests
+      env:
+        # Hack to get around this issue:
+        # https://stat.ethz.ch/pipermail/r-package-devel/2020q3/005930.html
+        #
+        # The system clock check during `R CMD check` relies on two external web APIs and fails
+        # when they are unavailable. By setting `_R_CHECK_SYSTEM_CLOCK_` to FALSE, we can skip it:
+        # https://github.com/wch/r-source/blob/59a1965239143ca6242b9cc948d8834e1194e84a/src/library/tools/R/check.R#L511
+        _R_CHECK_SYSTEM_CLOCK_: FALSE
       run: |
         export LINTR_COMMENT_BOT=false
         cd mlflow/R/mlflow/tests