ci: pin remaining zig runner images and log image in self-test

Pin ubuntu-latest -> ubuntu-24.04 and macos-latest -> macos-15 so
silent runner-image rollovers (which is what just broke
windows-latest) can't break this workflow without a deliberate bump.

Echo ImageOS/ImageVersion at the top of the self-test step so the
runner image is visible right next to any failure rather than buried
in the collapsed "Set up job" group.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: justsmth

.github/workflows/zig.yml

@@ -24,10 +24,10 @@ jobs:
           - host: windows-2022
             target_arch: x86_64
             target_system: Windows
-          - host: ubuntu-latest
+          - host: ubuntu-24.04
             target_arch: x86_64
             target_system: Linux
-          - host: macos-latest
+          - host: macos-15
             target_arch: aarch64
             target_system: Darwin
     steps:
@@ -68,6 +68,10 @@ jobs:
           # Each step runs unconditionally (no `set -e`) so we see results from
           # every stage even when an earlier one fails. The wrapper reports
           # zig's real exit status (including 139 for SIGSEGV) on failure.
+          # Log the runner image up front so a `*-latest` rollover (or our own
+          # pin bump) is correlated with any new failure without having to dig
+          # through the collapsed "Set up job" group.
+          echo "=== runner image: ${ImageOS:-unknown} ${ImageVersion:-unknown} ==="
           set -x
           probe() {
             echo "=== probe: $* ==="