Bump up the R snapshot date some more

yuvipanda · yuvipanda · commit 5572e37a7d94 · 2023-06-09T18:27:37.000+05:30
diff --git a/tests/r/r3.6-rspm/runtime.txt b/tests/r/r3.6-rspm/runtime.txt
@@ -1 +1 @@
-r-3.6-2018-11-20
+r-3.6-2018-12-20
