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

renovate.json

@@ -1,7 +1,12 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
   "extends": ["local>devxy/renovate-config"],
-  "ignorePaths": ["docker/**", ".crow/process-updates.yaml", ".crow/build-all-versions.yaml", ".crow/weekly-rebuild-missing.yaml"],
+  "ignorePaths": [
+    "docker/**",
+    ".crow/process-updates.yaml",
+    ".crow/build-all-versions.yaml",
+    ".crow/weekly-rebuild-missing.yaml"
+  ],
   "customManagers": [
     {
       "customType": "regex",
@@ -14,6 +19,13 @@
       "packageNameTemplate": "https://codefloe.com/rpkgs/bincraft.git",
       "datasourceTemplate": "git-tags",
       "extractVersionTemplate": "^v?(?<version>.+)$"
+    },
+    {
+      "customType": "regex",
+      "fileMatch": ["^local/uvr-install\\.sh$"],
+      "matchStrings": [
+        "# renovate: datasource=(?<datasource>\\S+) depName=(?<depName>\\S+)\\s+UVR_PIN=\"(?<currentValue>[^\"]+)\""
+      ]
     }
   ]
 }