[ci] [R-package] run 'R CMD check' as a foreground task (#6508)

.ci/test_r_package.sh

@@ -200,22 +200,10 @@ fi
 # fails tests if either ERRORs or WARNINGs are thrown by
 # R CMD CHECK
 check_succeeded="yes"
-(
-    R CMD check ${PKG_TARBALL} \
-        --as-cran \
-        --run-donttest \
-    || check_succeeded="no"
-) &
-
-# R CMD check suppresses output, some CIs kill builds after
-# a few minutes with no output. This trick gives R CMD check more time
-#     * https://github.com/travis-ci/travis-ci/issues/4190#issuecomment-169987525
-#     * https://stackoverflow.com/a/29890106/3986677
-CHECK_PID=$!
-while kill -0 ${CHECK_PID} >/dev/null 2>&1; do
-    echo -n -e " \b"
-    sleep 5
-done
+R CMD check ${PKG_TARBALL} \
+    --as-cran \
+    --run-donttest \
+|| check_succeeded="no"
 
 echo "R CMD check build logs:"
 BUILD_LOG_FILE=lightgbm.Rcheck/00install.out