Set the default CRAN mirror about planetary-computer-hub

[TomAugspurger]

Testing this out. We should be able to set the default easily enough (https://support.rstudio.com/hc/en-us/articles/360047157094-Managing-R-with-Rprofile-Renviron-Rprofile-site-Renviron-site-rsession-conf-and-repos-conf). Unfortunately it does look like there are binary incompatibilities, at least with the package I got (sf, links to libproj)

(notebook) jovyan@jupyter-taugspurger-40microsoft-2ecom:~$ cat /srv/conda/envs/notebook/lib/R/etc/Rprofile.site
options(
    repos = c(REPO_NAME = "https://packagemanager.rstudio.com/cran/__linux__/focal/latest"),
    HTTPUserAgent = "R/4.1.1 R (4.1.1 x86_64-pc-linux-gnu x86_64 linux-gnu)"
)
(notebook) jovyan@jupyter-taugspurger-40microsoft-2ecom:~$ R

R version 4.1.1 (2021-08-10) -- "Kick Things"
Copyright (C) 2021 The R Foundation for Statistical Computing
Platform: x86_64-conda-linux-gnu (64-bit)

R is free software and comes with ABSOLUTELY NO WARRANTY.
You are welcome to redistribute it under certain conditions.
Type 'license()' or 'licence()' for distribution details.

R is a collaborative project with many contributors.
Type 'contributors()' for more information and
'citation()' on how to cite R or R packages in publications.

Type 'demo()' for some demos, 'help()' for on-line help, or
'help.start()' for an HTML browser interface to help.
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[Previously saved workspace restored]

> install.packages("sf")
trying URL 'https://packagemanager.rstudio.com/cran/__linux__/focal/latest/src/contrib/sf_1.0-3.tar.gz'
Content type 'binary/octet-stream' length 8155842 bytes (7.8 MB)
==================================================
downloaded 7.8 MB

* installing *binary* package ‘sf’ ...
* DONE (sf)

The downloaded source packages are in
        ‘/tmp/RtmpoKfRG0/downloaded_packages’
Updating HTML index of packages in '.Library'
Making 'packages.html' ... done
> library(sf)
Error: package or namespace load failed for ‘sf’ in dyn.load(file, DLLpath = DLLpath, ...):
 unable to load shared object '/srv/conda/envs/notebook/lib/R/library/sf/libs/sf.so':
  libproj.so.15: cannot open shared object file: No such file or directory