feat(ci): OS/OS_VERSION manual dropdowns + restore lost crow fix (#97)

## Summary

Two changes:

1. **`OS`/`OS_VERSION` manual-run dropdowns** — give these form variables explicit `options:` lists (like `target_arch` and `R_VERSION`), so the manual-run form shows dropdowns instead of free-text, in both `build-all-versions.yaml` and `build-all-versions-install-deps.yaml`. Crow form variables are independent (no cascading), so the operator still has to pick a coherent `OS` + `OS_VERSION` combination (e.g. `redhat` + `9`, not `alpine` + `jammy`).

2. **Repairs `main`** — the crow fix from PR #96 (`bc2f6f1`) was lost when that PR was squashed (only the first commit was captured). As a result `main` currently carries the `OS: ${OS}` env vars that break Crow parsing (`unable to parse variable name`) and the unfixed `build-all.R`. This PR re-applies that fix: drop the env additions and derive `platform`/`arch` inside `build-all.R` from the container (bincraft codename → platform mapping + `Sys.info()` arch).

## Notes
- `OS_VERSION` options are quoted strings so tags like `8`/`9`/`10` aren't parsed as integers.
- Validated: both YAMLs parse, `build-all.R` parses.

Reviewed-on: #97

local/build-all.R

@@ -68,8 +68,31 @@ chunk <- chunk[!chunk$Package %in% exclude, ]
 # pkgs_to_build.rds is a static snapshot from the install-deps step, so on a
 # restart it still lists everything an interrupted run already produced. The
 # metadata DB reflects that progress, so we re-derive the remaining set here.
-platform <- paste(Sys.getenv("OS"), gsub("[.]", "", Sys.getenv("OS_VERSION")), sep = "-")
-arch <- Sys.getenv("ARCH")
+# Derive platform + arch from the running container, mirroring the codename ->
+# platform mapping bincraft uses internally. The OS/OS_VERSION selectors are
+# workflow-level CI variables that are not injected into the container
+# environment, so Sys.getenv() would return "" and this pre-filter would query
+# platform "-" and skip nothing.
+codename <- bincraft::set_codename(NULL)
+platform <- switch(
+  codename,
+  jammy = "ubuntu-2204",
+  noble = "ubuntu-2404",
+  resolute = "ubuntu-2604",
+  rhel10 = "redhat-10",
+  rhel9 = "redhat-9",
+  rhel8 = "redhat-8",
+  alpine320 = "alpine-320",
+  alpine321 = "alpine-321",
+  alpine322 = "alpine-322",
+  alpine323 = "alpine-323",
+  alpine324 = "alpine-324",
+  alpine325 = "alpine-325",
+  alpine326 = "alpine-326",
+  NA_character_
+)
+local_machine <- Sys.info()[["machine"]]
+arch <- if (grepl("arm64|aarch64", local_machine)) "arm64" else "amd64"
 con <- DBI::dbConnect(
   RPostgres::Postgres(),
   dbname = "build_metadata",