Add Windows code signing setup for x64 builds (#22022)

## Summary
- Implements automated Windows code signing for x64 and x64-baseline
builds
- Integrates DigiCert KeyLocker for secure certificate management
- Adds CI/CD pipeline support for signing during builds

## Changes
- Added `.buildkite/scripts/sign-windows.sh` script for automated
signing
- Updated CMake configurations to support signing workflow
- Modified build scripts to integrate signing step

## Testing
- Script tested locally with manual signing process
- Successfully signed test binaries at:
  - `C:\Builds\bun-windows-x64\bun.exe`
  - `C:\Builds\bun-windows-x64-baseline\bun.exe`

## References
Uses DigiCert KeyLocker tools for Windows signing

## Next Steps
- Validate Buildkite environment variables in CI
- Test full pipeline in CI environment

---------

Co-authored-by: Jarred Sumner <jarred@bun.sh>
Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

scripts/build.mjs

@@ -5,7 +5,9 @@ import { chmodSync, cpSync, existsSync, mkdirSync, readFileSync } from "node:fs"
 import { basename, join, relative, resolve } from "node:path";
 import {
   formatAnnotationToHtml,
+  getSecret,
   isCI,
+  isWindows,
   parseAnnotations,
   printEnvironment,
   reportAnnotationToBuildKite,
@@ -214,14 +216,47 @@ function parseOptions(args, flags = []) {
 async function spawn(command, args, options, label) {
   const effectiveArgs = args.filter(Boolean);
   const description = [command, ...effectiveArgs].map(arg => (arg.includes(" ") ? JSON.stringify(arg) : arg)).join(" ");
+  let env = options?.env;
+
   console.log("$", description);
 
   label ??= basename(command);
 
   const pipe = process.env.CI === "true";
+
+  if (isBuildkite()) {
+    if (process.env.BUN_LINK_ONLY && isWindows) {
+      env ||= options?.env || { ...process.env };
+
+      // Pass signing secrets directly to the build process
+      // The PowerShell signing script will handle certificate decoding
+      env.SM_CLIENT_CERT_PASSWORD = getSecret("SM_CLIENT_CERT_PASSWORD", {
+        redact: true,
+        required: true,
+      });
+      env.SM_CLIENT_CERT_FILE = getSecret("SM_CLIENT_CERT_FILE", {
+        redact: true,
+        required: true,
+      });
+      env.SM_API_KEY = getSecret("SM_API_KEY", {
+        redact: true,
+        required: true,
+      });
+      env.SM_KEYPAIR_ALIAS = getSecret("SM_KEYPAIR_ALIAS", {
+        redact: true,
+        required: true,
+      });
+      env.SM_HOST = getSecret("SM_HOST", {
+        redact: true,
+        required: true,
+      });
+    }
+  }
+
   const subprocess = nodeSpawn(command, effectiveArgs, {
     stdio: pipe ? "pipe" : "inherit",
     ...options,
+    env,
   });
 
   let killedManually = false;