feat(build): allow the per-minor pass to run under R 4.6 (#183)

## Motivation

The supported window is the latest R minor plus the two previous, which the build images install as `R_VERSION_LATEST=4.6.0`, `PREV1=4.5.3`, `PREV2=4.4.3`. This pipeline's `R_VERSION` offered only the latter two, so **no pipeline could run `local/build-all.R --sensitive-only` under 4.6** and its per-minor slots kept a backlog.

That backlog is the live bug. `rlang` is built for 4.4 and 4.5 on `amd64/resolute` but never for 4.6, so an R 4.6.1 client is served the generic 4.5.3 binary and dies with `undefined symbol: SETLENGTH`. 2709 records across the 16 slots are in that state.

## Why not weekly-rebuild-missing

I tried that first (#182) and it is the wrong tool, for two independent reasons:

- `weekly-missing-binaries-audit.R` reads only `/latest/src/contrib/PACKAGES.gz` and has no `r_minor` awareness, so its candidate list can only contain packages missing from the **generic** slot.
- `rebuild-missing.R:73` says it outright: *"rebuild passes no `is_r_minor_sensitive`, so it only ever targets the flat"*.

Running it under 4.6 built with the right interpreter and wrote to the wrong slot. It built almost nothing, and I verified it contaminated nothing: `amd64/resolute`'s flat slot is 22503 records at 4.5 and zero at 4.6. #182 should be closed.

`build-all-versions` already runs `--sensitive-only`, documented as "the extra per-minor passes under non-primary R versions". It only needed the option.

## Change

Adds `4.6.0` to `R_VERSION`. Default unchanged.

```sh
crow pipeline create devxy/build-cran-binaries \
  --var target_arch=amd64 --var OS=ubuntu --var OS_VERSION=resolute --var R_VERSION=4.6.0
```

## Follow-up worth doing separately

The audit has no per-minor awareness, so this gap is invisible to every existing check and will silently reopen. Nothing measures per-minor completeness today except `scripts/verify-r-minor-routing.sh`, which was written for routing rather than coverage.

Reviewed-on: #183

.crow/build-all-versions.yaml

@@ -36,7 +36,13 @@ variables:
     default: '3.24'
   R_VERSION:
     description: 'Primary R version under /opt/R.'
+    # The supported window is latest plus the two previous, which the build
+    # images install as R_VERSION_LATEST/PREV1/PREV2. 4.6.0 was missing here,
+    # so no pipeline could run the per-minor pass for it and its slots kept a
+    # backlog: rlang exists for 4.4 and 4.5 on amd64/resolute but not 4.6,
+    # which is how an R 4.6.1 client ended up loading a 4.5.3 binary.
     options:
+      - 4.6.0
       - 4.5.3
       - 4.4.3
     default: 4.5.3