ci: stop the Windows binary check rebuilding PHP on every run - #208

The job takes about 15 minutes. Profiling it:

  727s  Build static PHP binary
  153s  Combine PHP + PHAR -> binary
   30s  Install NASM
   11s  Smoke-test binary

The build dominates, and it should not be running at all. The Linux job caches
the same artefacts and its build step takes 1 second on a hit. The Windows
cache was never saved:

  Failed to save: Unable to reserve cache with key spc-win32-x64-v1-php8.3-...

so every run compiled PHP from scratch. The key included a hash of the whole
workflow file, which gives a different key per branch and a fresh rebuild for
any edit to the file — including edits to the Linux job or a comment.

The key is now over the four values that actually determine the build: the SPC
version, the PHP version, and the two extension lists. That is stable across
branches, so the cache is written once and hit by everything after it.

restore-keys are deliberately not used here. A prefix match would restore a
buildroot compiled with a different extension set, and the result would be a
binary quietly missing extensions — which is the failure this job exists to
catch.

The combine step piped a ~30 MB concatenation through PowerShell's object
pipeline one byte at a time. A stream copy does it in about a second.

The Linux job is untouched: its cache is restoring and its build takes 1s.
Rewriting a working fast path for consistency would risk more than it gains.

Expected: about 15 minutes on the first run that populates the cache, then
roughly 1-2 minutes.

Co-Authored-By: Claude <noreply@anthropic.com>