upgrade r-base to R 4.0.0 released yesterday

because R 4.0.0 requires a transition (which has not yet started) we (just
this once) pivot to experimental---and because r-recommended requires a good
dozen or so "recommended" packages from CRAN (all maintained by R Core too)
I have placed binaries for these into a PPA-alike repo on GitHub

library/r-base

@@ -2,7 +2,7 @@ Maintainers: Carl Boettiger <[email protected]> (@cboettig),
              Dirk Eddelbuettel <[email protected]> (@eddelbuettel)
 GitRepo: https://github.com/rocker-org/rocker.git
 
-Tags: 3.6.3, latest
+Tags: 4.0.0, latest
 Architectures: amd64, arm64v8
-GitCommit: e76a46e4d082c0294437c34df1072f48c6fa3685
+GitCommit: 1286d1fa0165a06b93ea9c4cebd94a54cbf86227
 Directory: r-base