[MINOR][R] add SparkR.Rcheck/ and SparkR_*.tar.gz to R/.gitignore

## What changes were proposed in this pull request?

Ignore temp files generated by `check-cran.sh`.

Author: Xiangrui Meng <meng@databricks.com>

Closes #14740 from mengxr/R-gitignore.

R/.gitignore

@@ -4,3 +4,5 @@
 lib
 pkg/man
 pkg/html
+SparkR.Rcheck/
+SparkR_*.tar.gz