refactor(ci): consolidate build-all-versions workflows into 2 files (#93)

## Summary

Consolidate the four `build-all-versions*` files into two, mirroring the matrix approach used for the process-updates / weekly families:

| Before | After |
|---|---|
| `build-all-versions-amd64.yaml` + `build-all-versions-arm64.yaml` | `build-all-versions.yaml` |
| `build-all-versions-install-deps-amd64.yaml` + `-arm64.yaml` | `build-all-versions-install-deps.yaml` |

`ARCH` becomes a matrix axis carrying the per-arch differences: `agent` (artemis/gaia), `BACKEND` (kubernetes/docker), and `R_PKG_CACHE_DIR`. Build-all keeps its `SPLIT_INDEX` parallelism, so its matrix is ARCH x SPLIT = 8 rows; only the rows matching the selected arch run.

**Invocation unchanged.** Routing uses the existing task selector `task == "build-all-${ARCH}"`, so `--var task=build-all-amd64 ...` still runs exactly the amd64 rows. `OS` / `OS_VERSION` / `R_VERSION` / `K8S_INSTANCE_TYPE` are still passed via `--var`. (You can drop `--var ARCH=...` now — arch comes from the matrix/task.)

**No cron.** These pipelines are manual-only, so there are no scheduled runs to affect.

## Validation before merge

Same matrix-interpolation-in-`when`/`labels` mechanism validated for #92, plus it now relies on per-row `agent`/`BACKEND` selecting the right execution backend (kubernetes for amd64, docker for arm64). Before merge, on this branch trigger a small `task=build-all-arm64` run and confirm it lands on the gaia/docker backend (and amd64 on artemis/kubernetes). Originals are in git history for rollback.

Note: `archive-missed-packages.yaml` was already a single consolidated matrix file (cron + manual) and is left untouched.
Reviewed-on: #93

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

@@ -0,0 +1,75 @@
+# Consolidated install-deps for build-all (both arches in one file).
+# Triggered together with build-all-versions; `target_arch` routes the matrix.
+# Variables are declared so the manual-run form exposes them (crow #1165);
+# they are merged with build-all-versions' identical declarations.
+variables:
+  target_arch:
+    description: "Architecture to build."
+    options:
+      - amd64
+      - arm64
+    default: amd64
+  OS:
+    description: "Base OS image name (e.g. alpine, redhat, ubuntu)."
+    default: alpine
+  OS_VERSION:
+    description: "OS version / image tag (e.g. 3.24, 9, jammy, noble)."
+    default: "3.24"
+  R_VERSION:
+    description: "Primary R version under /opt/R."
+    options:
+      - 4.5.3
+      - 4.4.3
+    default: 4.5.3
+
+when:
+  - event: manual
+    evaluate: 'target_arch == "${ARCH}"'
+
+skip_clone: true
+
+labels:
+  platform: linux/${ARCH}
+  agent: ${AGENT}
+
+matrix:
+  include:
+    - ARCH: amd64
+      AGENT: artemis
+    - ARCH: arm64
+      AGENT: gaia
+
+steps:
+  - name: 'Install deps and bincraft'
+    image: 'reg.devxy.io/rpkgs/build-env-${OS}:${OS_VERSION}'
+    pull: true
+    environment:
+      REPO_RO_TOKEN:
+        from_secret: REPO_RO_TOKEN
+      GITHUB_PAT:
+        from_secret: GITHUB_PAT
+      B2_S3_ACCESS_KEY:
+        from_secret: B2_S3_ACCESS_KEY
+      B2_S3_SECRET_KEY:
+        from_secret: B2_S3_SECRET_KEY
+      PGPASS:
+        from_secret: PGPASS
+      R_PKG_CACHE_DIR: /mnt/cache/pkgcache
+      R_LIBS_USER: /mnt/cache/R-pkgs
+      CCACHE_DIR: /mnt/cache/ccache
+    volumes:
+      - ${ARCH}-binaries-r-dep-cache-${OS}-${OS_VERSION//./}:/mnt/cache
+    commands:
+      # one-time full wipe to fix corrupted .so files from previous failed builds
+      # - rm -rf /mnt/cache/R-pkgs
+      - mkdir -p /mnt/cache/pkgcache /mnt/cache/R-pkgs /mnt/cache/ccache /mnt/cache/packages
+      - git clone -q https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
+      - git clone -q https://codefloe.com/rpkgs/bincraft.git /tmp/bincraft
+      - /opt/R/$R_VERSION/bin/R -q -e 'pak::sysreqs_db_update(); pak::local_install("/tmp/bincraft"); pak::pak(c("RPostgres", "s3fs", "data.table", "future", "jsonlite")); packageVersion("bincraft")'
+      - /opt/R/$R_VERSION/bin/R -q -e "source('local/packages-to-build.R'); saveRDS(pkgs, '/mnt/cache/packages/pkgs_to_build.rds'); saveRDS(pkgs[r_minor_sensitive == TRUE], '/mnt/cache/packages/r_minor_sensitive_pkgs.rds'); sprintf('Precomputed %s package versions (%s r-minor-sensitive)', nrow(pkgs), nrow(pkgs[r_minor_sensitive == TRUE]))"
+    backend_options:
+      docker:
+        resources:
+          requests:
+            memory: 5Gi
+            cpu: 1000m