ci: give the Windows test job real timeout headroom (#717)

The `test` job ran with `timeout-minutes: 12` while a green Windows run
takes 11.8 minutes, leaving about 12 seconds of margin. Runner variance,
not the code under review, decided the outcome: PR #711's rerun finished
at 11.8min and passed, PR #653's was killed at 12.0min with `Test`
cancelled and every later step skipped.

That flakiness is expensive to review around, because `gh pr checks`
renders a cancelled job as `fail`. A red Windows check could mean a real
failure, a `cancel-in-progress` concurrency cancel, or this timeout, and
telling them apart requires reading `conclusion` from the check-runs API
plus per-job timings.

Measured on run 30459554635 (same suite, same commit):

  ubuntu-latest    4.6 min
  macos-latest     5.6 min
  windows-latest  11.8 min

20 minutes keeps a green Windows run green with margin. It does not
address why Windows needs ~2.5x Linux on an identical suite; the comment
says so explicitly, because the ceiling has already been raised once
(8 -> 12) for this same reason and chasing it again is not a fix. The
`npm-global-smoke` job is untouched at 8 minutes: it completes in 1-2
minutes and was never near its limit.

Refs #717

Author: lidge-jun

.github/workflows/ci.yml

@@ -48,9 +48,17 @@ jobs:
   test:
     name: ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
-    # The Windows full suite now completes near the old 8-minute ceiling; leave
-    # enough room for the privacy/build smokes instead of cancelling a green run.
-    timeout-minutes: 12
+    # Windows dominates this matrix: on run 30459554635 the same suite took
+    # ubuntu 4.6min / macos 5.6min / windows 11.8min. Against the previous
+    # 12-minute ceiling that left ~12s of headroom, so runner variance decided
+    # the result rather than the code under review — #711's rerun finished at
+    # 11.8min and passed while #653's was killed at 12.0min (issue #717).
+    # A cancelled job renders as `fail` in `gh pr checks`, so that flakiness
+    # reads as a broken PR. 20 minutes keeps a green Windows run green with
+    # real margin; it is not a licence for the suite to grow into it. If
+    # Windows approaches this ceiling too, fix the 2.5x platform gap instead
+    # of raising the number again.
+    timeout-minutes: 20
     strategy:
       fail-fast: false
       matrix: