Avoid caching the last few days in case new runs appear

This regressed with web-platform-tests#87, but it doesn't occur in CI as we never have a
populated cache when this is run.

lib/runs.js

@@ -89,6 +89,7 @@ async function fetchAlignedRunsFromServer(products, from, to, experimental) {
 
   let cachedCount = 0;
   const before = moment();
+  const noCacheAfter = moment().sub('3', 'days');
   const alignedRuns = new Map();
   while (from < to) {
     const formattedFrom = from.format('YYYY-MM-DD');
@@ -113,10 +114,14 @@ async function fetchAlignedRunsFromServer(products, from, to, experimental) {
       // No cache hit; load from the server instead.
       const url = `${runsUri}&from=${formattedFrom}&to=${formattedTo}`;
       const response = await fetch(url);
-      // Many days do not have an aligned set of runs, but we always write to
-      // the cache to speed up future executions of this code.
       runs = await response.json();
-      await fs.promises.writeFile(cacheFile, JSON.stringify(runs));
+
+      if (from.isSameOrBefore(noCacheAfter)) {
+        // Avoid caching for the last few days, as new runs may still appear
+        // here; otherwise, cache unconditionally, even if we do not have an
+        // aligned set of runs.
+        await fs.promises.writeFile(cacheFile, JSON.stringify(runs));
+      }
     }
 
     if (!runs.length) {