refactor: migrate package installation from pak to uvr

bincraft dropped pak in favour of uvr, so the pipelines, helper scripts
and images in this repo move with it.

- add local/uvr-install.sh as the single replacement for pak::pak(); it
  bootstraps a pinned uvr, mints a throwaway project under TMPDIR and
  runs `uvr add --no-install` + `uvr sync --library`, because `uvr add`
  refuses to run outside a project and only `sync` honours --library
- install bincraft via its Forgejo spec (forgejo::codefloe.com/rpkgs/
  bincraft@<tag>) instead of a git:: URL, keeping the git ls-remote tag
  resolution
- pass UVR_R_BIN/UVR_TARGET_LIB from install-bincraft.R so the
  per-R-minor passes target their own R and library
- replace R_PKG_CACHE_DIR with UVR_CACHE_DIR/UVR_PACKAGES_DIR on the
  persistent volume, preserving the amd64-off/arm64-on split
- drop trim_pkgcache_metadata() and its test; uvr's cache does not grow
  the way pkgcache's _metadata dir did
- let uvr install system requirements from its vendored
  r-system-requirements rules, replacing pak::sysreqs_db_update()
- migrate the shiny app image, the alpine reprex and the CRAN loading
  test; ship uvr-install.sh in the build-one image
- track the uvr pin with renovate

docker/build-one.Dockerfile

@@ -17,7 +17,10 @@ ARG CACHEBUST
 WORKDIR /work
 COPY build-one.R /work/build-one.R
 # Resolve and install the latest bincraft release dynamically (no hardcoded pin).
+# uvr-install.sh lands under /work/local/ because install-bincraft.R looks for it
+# there when the working directory is not a repo checkout.
 COPY install-bincraft.R /work/install-bincraft.R
+COPY uvr-install.sh /work/local/uvr-install.sh
 # Ship the patch registry so build-one.R's `patches = "local/patches"` resolves
 # (build context is `local/`, CWD is /work).
 COPY patches /work/local/patches