Partial update MRAN dates

Add libpcre2-dev as it is needed by latest r-devel

When the image is r-ver:devel is built without libpcre2-dev,
the following issue is generated.

```
checking whether PCRE support suffices... no
configure: error: PCRE2 library and headers are required, or use --with-pcre1 and PCRE >= 8.32 with UTF-8 support
/bin/sh: 1: Rscript: not found
```

This allows a clean build

Use https to download the .deb file if RSTUDIO_VERSION is provided

3.6.2

Patch error handling of tlmgr path in TeX Live 2016

* See https://tex.stackexchange.com/a/314079.
* Apply the corresponding fix https://git.texlive.info/texlive/commit/Master/tlpkg/TeXLive/TLUtils.pm?id=69cee5e1ce4b20f6ebb6af77e19d49706a842a3e instead of the `||true` workaround from #171.
* Fixes /rocker-org/rocker#370.

Fix site-library permissions with debian:buster

* makes all of its content inherit the same group (staff) upon creation
* needed with debian:buster, which does not set this for the whole /usr/local anymore

Do not override externally set R_LIBS_USER

Fixes #175

Patch the Perl modules in the frozen TeX Live 2018 snapshot

* Use the newer version available for the installer in tlnet/tlpkg/TeXLive, to include the fix described in https://github.com/yihui/tinytex/issues/77#issuecomment-466584510
* As discussed in https://www.preining.info/blog/2019/09/tex-services-at-texlive-info/#comments
* Optional patch only applied for TeX Live 2018, currently relevant for verse:devel but also backported to the verse:latest to be ready for next R release with the update to debian:buster

Debian buster needs to use ubuntu bionic build of RStudio, which is compiled against the more recent (1.1) version of libssl

Version-stable CTAN repo for the TinyTeX install

* Fixes #169.
* TeXLive release bound to the base Debian image.
* CTAN repo exposed as build argument.
* Backported down to verse:3.4.3.
* Use http for TeXLive 2016: install-tl did not support https repositories.
* Work around known non-breaking errors of `tlmgr path add` in TeXLive 2016 (https://tex.stackexchange.com/a/314079).

added system package devscripts to r-ver/devel Dockerfile

the package devscripts contains the script checkbashisms which is now required by R CMD check --as-cran