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/test-package-loading.R

@@ -1,22 +1,28 @@
-install.packages(
-  "pak",
-  repos = sprintf(
-    "https://r-lib.github.io/p/pak/stable/%s/%s/%s",
-    .Platform$pkgType,
-    R.Version()$os,
-    R.Version()$arch
+# Installs every CRAN package one by one and checks that it loads. Dependencies
+# go through uvr via local/uvr-install.sh, which bootstraps the uvr binary on
+# first use and installs into .libPaths()[1].
+uvr_install <- function(pkg) {
+  Sys.setenv(
+    UVR_R_BIN = file.path(R.home("bin"), "R"),
+    UVR_TARGET_LIB = .libPaths()[1L]
   )
-)
+  status <- system2("local/uvr-install.sh", shQuote(pkg))
+  if (!identical(status, 0L)) {
+    stop(
+      sprintf("uvr failed to install %s (exit %s)", pkg, status),
+      call. = FALSE
+    )
+  }
+}
 
-Sys.setenv(PKG_SYSREQS = TRUE)
 all_pkgs <- rownames(available.packages())
 
-to_skip = c("ABRSQOL", "ACA", "ACE.CoCo")
-all_pkgs = setdiff(all_pkgs, to_skip)
+to_skip <- c("ABRSQOL", "ACA", "ACE.CoCo")
+all_pkgs <- setdiff(all_pkgs, to_skip)
 
 for (i in all_pkgs) {
   message(sprintf("\nInstalling %s", i))
-  pak::pkg_install(i)
+  uvr_install(i)
   library(i, character.only = TRUE)
 }
 
@@ -323,7 +329,7 @@ if (length(to_process) == 0) {
 } else {
   for (i in to_process) {
     message(sprintf("\nInstalling %s", i))
-    pak::pkg_install(i)
+    uvr_install(i)
     library(i, character.only = TRUE)
   }
   # Update to_skip to include all up to the last processed