fix(build): scope the already-attempted skip to the running R minor (#187)

## Motivation

The run meant to close the 4.6 gap on `amd64/resolute` barely built anything:

```
[1] "Skipped 2334 already-attempted package versions; 59 remaining for this job"
```

`single_builds` records `r_version` per attempt — `store_build_metadata()` both writes and queries it — but the skip query here ignored that column:

```sql
SELECT name, tag FROM single_builds WHERE platform = $1 AND arch = $2
```

So a non-primary pass skipped every package the **primary** pass had already attempted under a different minor. `build-all.R --sensitive-only` running under R 4.6 skipped packages that had only ever been built for 4.5.

That is the reason the per-minor slots never fill, and why the backlog cannot be worked off by rebuilding: `amd64/resolute` serves a 4.6 client 22322 packages against the 4.5 slot's 26346.

It is also, ultimately, why an R 4.6.1 client got a 4.5-built `rlang` and `undefined symbol: SETLENGTH`. Every other fix in this chain addressed a consequence; this is the cause.

## Change

Scope the skip to the R minor the pass is running under.

Matched on the `major.minor` prefix rather than the full `r_version` string, so a patch bump (4.6.0 → 4.6.1) does not re-attempt the entire catalogue. Verified the prefix extraction against `4.5.3`, `4.6.0`, `4.4.3` and a bare `4.6`, and that the derivation matches what `store_build_metadata()` records.

## Expected effect

The non-primary passes stop skipping wholesale. The first run per slot will be long, since it works off a backlog that has been accumulating for as long as the per-minor slots have existed.

## Verification

- `local/build-all.R` parses.
- Minor derivation checked under R 4.6.1: `4.6`.
- Real effect is only observable from a run; the number to watch is the "Skipped N ... M remaining" line, which should show a far larger `M` for a non-primary pass.

Reviewed-on: #187

local/build-all.R

@@ -110,10 +110,25 @@ con <- DBI::dbConnect(
   password = Sys.getenv("PGPASS"),
   sslmode = "require"
 )
+# Scope the skip to the R minor this pass is running under. `single_builds`
+# records `r_version` per attempt, but querying without it made a non-primary
+# pass skip everything the primary pass had already attempted under a different
+# minor - so `--sensitive-only` under 4.6 skipped packages that had only ever
+# been built for 4.5, and the per-minor slots never filled. That is why
+# amd64/resolute served 4000 fewer packages to a 4.6 client than to a 4.5 one.
+r_minor <- paste(
+  R.version$major,
+  strsplit(R.version$minor, ".", fixed = TRUE)[[1L]][1L],
+  sep = "."
+)
 built <- DBI::dbGetQuery(
   con,
-  "SELECT name, tag FROM single_builds WHERE platform = $1 AND arch = $2",
-  params = list(platform, arch)
+  paste(
+    "SELECT name, tag FROM single_builds",
+    "WHERE platform = $1 AND arch = $2",
+    "AND substring(r_version from '^[0-9]+[.][0-9]+') = $3"
+  ),
+  params = list(platform, arch, r_minor)
 )
 DBI::dbDisconnect(con)
 before <- nrow(chunk)
@@ -121,8 +136,9 @@ chunk <- chunk[
   !paste(chunk$Package, chunk$Version) %in% paste(built$name, built$tag),
 ]
 sprintf(
-  "Skipped %d already-attempted package versions; %d remaining for this job",
+  "Skipped %d package versions already attempted under R %s; %d remaining for this job",
   before - nrow(chunk),
+  r_minor,
   nrow(chunk)
 )