fix: check payload-dumper-go's exec bit without /usr/bin/test

The macOS packaging job died with "/usr/bin/test is not recognized". `test`
lives at /bin/test on macOS; /usr/bin/test is a Linux path. Only the App
osx-arm64 job reached this branch, so it was the only one to fail.

Uses System.IO.File.GetUnixFileMode instead of shelling out, which needs no
external binary and no assumption about its path.

Reproduced the failure and verified the fix locally under pwsh on macOS,
including stripping the exec bit to confirm the check still rejects a
non-executable payload-dumper-go.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: backspace135

.github/workflows/publish.yml

@@ -255,9 +255,9 @@ jobs:
             }
 
             $payloadPath = Join-Path $extractDir "${appPrefix}root-tools/payload-dumper/$payloadExecutable"
-            & /usr/bin/test -x $payloadPath
-            if ($LASTEXITCODE -ne 0) {
-              throw "macOS payload-dumper-go is not executable in '$($zip.Name)'."
+            $mode = [System.IO.File]::GetUnixFileMode($payloadPath)
+            if (-not ($mode -band [System.IO.UnixFileMode]::UserExecute)) {
+              throw "macOS payload-dumper-go is not executable in '$($zip.Name)' (mode: $mode)."
             }
           }