Refactor parameter handling in build and sign scripts for PowerShell 5.1 compatibility

sign-caffeine-exe.ps1

@@ -1,6 +1,6 @@
 [CmdletBinding()]
 param(
-    [string]$ExePath = (Join-Path $PSScriptRoot 'dist\PortableCaffeine.exe'),
+    [string]$ExePath,
     [string]$Subject = 'CN=Portable Caffeine Dev Code Signing',
     [string]$FriendlyName = 'Portable Caffeine Dev Code Signing (Self-Signed)',
     [ValidateSet('None', 'CurrentUser', 'LocalMachine')]
@@ -10,7 +10,7 @@ param(
     [int]$YearsValid = 5,
     [switch]$ForceNewCertificate,
     [string]$TimestampServer,
-    [string]$CerExportPath = (Join-Path $PSScriptRoot 'dist\PortableCaffeine-dev-signing.cer'),
+    [string]$CerExportPath,
     [string]$PfxExportPath,
     [securestring]$PfxPassword
 )
@@ -18,6 +18,23 @@ param(
 Set-StrictMode -Version Latest
 $ErrorActionPreference = 'Stop'
 
+# In Windows PowerShell 5.1, $PSScriptRoot may be empty during param default evaluation.
+# Resolve script-relative defaults after the param block for compatibility.
+$scriptRoot = if (-not [string]::IsNullOrWhiteSpace($PSScriptRoot)) {
+    $PSScriptRoot
+} elseif ($MyInvocation.MyCommand.Path) {
+    Split-Path -Parent $MyInvocation.MyCommand.Path
+} else {
+    (Get-Location).Path
+}
+
+if ([string]::IsNullOrWhiteSpace($ExePath)) {
+    $ExePath = Join-Path $scriptRoot 'dist\PortableCaffeine.exe'
+}
+if ([string]::IsNullOrWhiteSpace($CerExportPath)) {
+    $CerExportPath = Join-Path $scriptRoot 'dist\PortableCaffeine-dev-signing.cer'
+}
+
 if ([Environment]::OSVersion.Platform -ne [PlatformID]::Win32NT) {
     throw "This script is intended for Windows only."
 }