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

local/packages-to-build.R

@@ -15,15 +15,15 @@ suppressPackageStartupMessages(library(data.table))
 # Sys.setenv("OS_VERSION" = "3.22")
 # Sys.setenv("ARCH" = "arm64")
 
-arch = Sys.getenv("ARCH")
+arch <- Sys.getenv("ARCH")
 # target: alpine-322, ubuntu-2404, redhat-9, etc.
-platform = paste(
+platform <- paste(
   Sys.getenv("OS"),
   gsub("[.]", "", Sys.getenv("OS_VERSION")),
   sep = "-"
 )
 # Use bincraft's codename detection for S3 paths (e.g. "rhel10" not "redhat10")
-codename = bincraft::set_codename(NULL)
+codename <- bincraft::set_codename(NULL)
 
 con <- DBI::dbConnect(
   RPostgres::Postgres(),
@@ -35,8 +35,8 @@ con <- DBI::dbConnect(
   sslmode = "require"
 )
 
-cran_archive = tools::CRAN_archive_db()
-cran_release = tools::CRAN_package_db()
+cran_archive <- tools::CRAN_archive_db()
+cran_release <- tools::CRAN_package_db()
 # Subset cran_archive to only those packages
 cran_archive_in_release <- cran_archive[
   names(cran_archive) %in% cran_release$Package
@@ -84,13 +84,14 @@ s3fs::s3_file_system(
   region_name = "eu-central-003",
   refresh = TRUE
 )
-s3_pkgs = s3fs::s3_dir_ls(
+s3_pkgs <- s3fs::s3_dir_ls(
   sprintf("devxy-rpkgs-binaries/%s/%s/latest/src/contrib", arch, codename),
   recurse = TRUE
 )
 
 # Save the raw S3 file listing for the build step to use as s3_package_cache
-# This avoids loading s3fs/reticulate in the build container, saving memory for pak forks
+# This avoids loading s3fs/reticulate in the build container, saving memory for
+# the dependency-installer subprocesses
 saveRDS(basename(s3_pkgs), "/mnt/cache/packages/s3_cache.rds")
 
 file_names <- basename(s3_pkgs)