jenkins: build tarballs on tag

[WardBrian]

We have some helper scripts for this over at https://github.com/stan-dev/ci-scripts/tree/master/release-scripts

Pseudocode:

if BUILDING_TAG:
  download stanc3 binaries from https://github.com/stan-dev/stanc3/releases/tag/$TAG
  set VERSION = TAG without preceeding 'v'
  rename cloned folder to cmdstan-$VERSION
  copy linux-stanc, macos-stanc, and windows-stanc to cmdstan-$VERSION/bin
  run "tar --exclude-vcs --hard-dereference -chzvf cmdstan-$VERSION.tar.gz cmdstan-$VERSION/"

  then, run the rest of https://github.com/stan-dev/ci-scripts/blob/master/release-scripts/build-non-x86-tarballs.sh#L12-L35

cc @dylex