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

The supported window is the latest R minor plus the two previous, which
the build images install as R_VERSION_LATEST/PREV1/PREV2 (4.6.0, 4.5.3,
4.4.3). R_VERSION here 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'.

weekly-rebuild-missing cannot close it - its audit reads only the generic
index and rebuild-missing.R targets only the flat slot - so this is the
pipeline that has to grow the option.

.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