Update CRAN key to recover docker image generation

CRAN repo changed the key.

• https://cran.r-project.org/bin/linux/ubuntu/README.html

  Err:1 https://cloud.r-project.org/bin/linux/ubuntu bionic-cran35/ InRelease
    The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9
  ...
  W: GPG error: https://cloud.r-project.org/bin/linux/ubuntu bionic-cran35/ InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9
  E: The repository 'https://cloud.r-project.org/bin/linux/ubuntu bionic-cran35/ InRelease' is not signed.
  

Although they changed the key, but it reuses `cran35` for R 3.6.

Even though R has moved to version 3.6, for compatibility the sources.list entry still uses the cran3.5 designation.

This issue aims to recover the docker image generation first. We will verify the R doc generation in a separate issue.