fix(ci): do not print a missing timing as "NAs" (#210)

## Why

A `MISSING_FROM_INDEX` row has no install to time, and `sprintf("%.1fs", NA_real_)` renders it as the literal `NAs`:

```
ROW  arch  amd64  alpine323  4.5  Rcpp  MISSING_FROM_INDEX  |NAs
```

Read quickly, `NAs` looks like a unit rather than a missing measurement.

## What changed

Rows without a timing print the `built` field alone. Every other row is unchanged.

## Where it showed up

Pipeline 12492, the deliberate failure-path check against a slot known to have gaps. That run is otherwise a good result: it caught `Rcpp` missing from the alpine323 4.5 and 4.6 indexes on both contractual URL forms, and it caught the reverse-direction ABI break on the flat slot (objects built under R 4.5.3 failing `dyn.load()` under 4.4), which no previous reprex had surfaced.

Reviewed-on: #210

local/verify-repo-routing.R

@@ -96,7 +96,10 @@ emit <- function(package, status, built = "", secs = NA_real_) {
       r_minor,
       package,
       status,
-      sprintf("%s|%.1fs", built, secs)
+      # `%.1f` renders NA as the literal "NAs", which reads as a unit rather
+      # than as a missing measurement. A package that was never in the index
+      # has no timing at all.
+      if (is.na(secs)) built else sprintf("%s|%.1fs", built, secs)
     )
   )
 }