ci(nightly): build Windows OpenSSL with Strawberry perl, not msys perl (closes #614)

[noahgift]

The nightly x86_64-pc-windows-msvc leg runs ./Configure for vendored OpenSSL under shell: bash, which resolves Git's msys perl. That perl lacks Params::Check (BEGIN failed--compilation aborted at /usr/share/perl5/core_perl/Params/Check.pm, run 2026-09-25 09:31Z). openssl-sys fails, the leg is red, and the all-or-nothing release job is skipped. The nightly tag is now 290h behind main, and fleet-bins reports NIGHTLY-STALE on every hourly lambda run.

This change sets OPENSSL_SRC_PERL to the Strawberry perl that windows-latest ships. The step also fails loudly if that perl cannot load the module the build needs, so a future image change stays visible.

Not verified before merge: a branch workflow_dispatch of nightly.yml would publish the nightly release from the branch commit, so I did not dispatch. The first scheduled run after merge is the proof.

Pmat-Ticket: PMAT-614

🤖 Generated with Claude Code

[noahgift]

quorum-review (AD-04): NOT agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-614",
 "head": "f78aef4305bacae8c3ea4750ced7db0d392a9fdb",
 "width": 3,
 "executor": "agy",
 "agreed": false,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "FAIL",
   "findings": 1
  },
  {
   "lane": 2,
   "verdict": "FAIL",
   "findings": 1
  },
  {
   "lane": 3,
   "verdict": "FAIL",
   "findings": 2
  }
 ]
}

[noahgift]

quorum-review (AD-04): three PASS — agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-614",
 "head": "215f2e8a2edffa0e2cacc4109cafd3bee559b30b",
 "width": 3,
 "executor": "agy",
 "agreed": true,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 4
  },
  {
   "lane": 2,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 4
  }
 ]
}