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

.crow/build-all-versions-install-deps.yaml

@@ -10,10 +10,21 @@ variables:
       - arm64
     default: amd64
   OS:
-    description: "Base OS image name (e.g. alpine, redhat, ubuntu)."
+    description: "Base OS image name."
+    options:
+      - alpine
+      - redhat
+      - ubuntu
     default: alpine
   OS_VERSION:
-    description: "OS version / image tag (e.g. 3.24, 9, jammy, noble)."
+    description: "OS image tag. Must match OS (alpine: 3.24; redhat: 8/9/10; ubuntu: jammy/noble)."
+    options:
+      - "3.24"
+      - "8"
+      - "9"
+      - "10"
+      - "jammy"
+      - "noble"
     default: "3.24"
   R_VERSION:
     description: "Primary R version under /opt/R."

.crow/build-all-versions.yaml

@@ -11,10 +11,21 @@ variables:
       - arm64
     default: amd64
   OS:
-    description: "Base OS image name (e.g. alpine, redhat, ubuntu)."
+    description: "Base OS image name."
+    options:
+      - alpine
+      - redhat
+      - ubuntu
     default: alpine
   OS_VERSION:
-    description: "OS version / image tag (e.g. 3.24, 9, jammy, noble)."
+    description: "OS image tag. Must match OS (alpine: 3.24; redhat: 8/9/10; ubuntu: jammy/noble)."
+    options:
+      - "3.24"
+      - "8"
+      - "9"
+      - "10"
+      - "jammy"
+      - "noble"
     default: "3.24"
   R_VERSION:
     description: "Primary R version under /opt/R."
@@ -90,13 +101,6 @@ steps:
         from_secret: GITHUB_PAT
       # normal env vars
       GIT_USER: pat-s
-      # Export the platform selectors as runtime env vars. They are otherwise
-      # only available for ${...} interpolation (image/volume), so build-all.R's
-      # `Sys.getenv("OS")/("OS_VERSION")/("ARCH")` would be empty and its
-      # already-built dedup query would match platform "-" and skip nothing.
-      OS: ${OS}
-      OS_VERSION: ${OS_VERSION}
-      ARCH: ${ARCH}
       # set the location of the 'pkgcache' cache dir which persists the R package dependencies needed to install the packages themselves
       R_PKG_CACHE_DIR: ${R_PKG_CACHE_DIR}
       R_LIBS_USER: /mnt/cache/R-pkgs