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/r-minor-helpers.R

@@ -31,42 +31,3 @@ parse_build_args <- function(args) {
     ncpus = as.integer(pos[3L])
   )
 }
-
-# Bound the {pkgcache} metadata dir, which otherwise grows without limit: the
-# "patched" repo mints a new content hash on every PACKAGES change, so each build
-# writes a fresh ~70 MB _metadata/pkgs-<hash>.rds (+ patched-<hash>/) that is
-# never reused. Keep the `keep` newest entries by mtime; only remove entries
-# older than `min_age_secs`, so a concurrent split-job's in-flight files are
-# never deleted (each build uses a unique hash, so aged entries are
-# unreferenced). Stable repo dirs (CRAN-*, BioC*, INLA-*) and pkg/ downloads are
-# not matched and thus preserved. Returns the number of entries removed.
-trim_pkgcache_metadata <- function(cache_dir = Sys.getenv("R_PKG_CACHE_DIR"),
-                                   keep = 20L,
-                                   min_age_secs = 600) {
-  meta <- file.path(cache_dir, "R", "pkgcache", "_metadata")
-  if (!nzchar(cache_dir) || !dir.exists(meta)) {
-    return(0L)
-  }
-  entries <- c(
-    Sys.glob(file.path(meta, "patched-*")),
-    Sys.glob(file.path(meta, "pkgs-*.rds"))
-  )
-  if (length(entries) == 0L) {
-    return(0L)
-  }
-  info <- file.info(entries)
-  order_new_first <- order(info$mtime, decreasing = TRUE)
-  ranked <- entries[order_new_first]
-  ranked_mtime <- info$mtime[order_new_first]
-  if (length(ranked) <= keep) {
-    return(0L)
-  }
-  candidates <- ranked[(keep + 1L):length(ranked)]
-  candidate_age <- as.numeric(Sys.time()) - as.numeric(ranked_mtime[(keep + 1L):length(ranked)])
-  removable <- candidates[candidate_age >= min_age_secs]
-  if (length(removable) == 0L) {
-    return(0L)
-  }
-  unlink(removable, recursive = TRUE, force = TRUE)
-  length(removable)
-}