Bump up the R snapshot date some more

2023-06-09 18:27:37 +05:30 · 2023-06-09 18:27:37 +05:30 · 5572e37a7d
commit 5572e37a7d
--- 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