fix(tests): remove PS6+ Start-Job -WorkingDirectory for 5.1 compat

The build.tests.ps1 Compile/Dot-sourced contexts started Wait-Job builds
with Start-Job -WorkingDirectory, a parameter introduced in PowerShell 6.0
that does not exist on Windows PowerShell 5.1. Under the shared workflow's
full 5.1 test run this threw ParameterBindingException, the TestModule never
built, and all 15 downstream assertions failed. The scriptblock already
Set-Location's to the test module source, so -WorkingDirectory was redundant;
removing it is behavior-identical on pwsh and 5.1-safe.

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

Author: tablackburn

tests/build.tests.ps1

@@ -21,11 +21,13 @@ Describe 'Build' {
             Write-Host "OutputPath: $script:testModuleOutputPath"
 
             # build is PS job so psake doesn't freak out because it's nested
+            # NOTE: the scriptblock Set-Location handles the working directory;
+            # Start-Job -WorkingDirectory is PS 6+ only and breaks on Windows PowerShell 5.1.
             Start-Job -Scriptblock {
                 Set-Location -Path $using:testModuleSource
                 $global:PSBuildCompile = $true
                 ./build.ps1 -Task Build
-            } -WorkingDirectory $script:testModuleSource | Wait-Job
+            } | Wait-Job
         }
 
         AfterAll {
@@ -73,11 +75,13 @@ Describe 'Build' {
     Context 'Dot-sourced module' {
         BeforeAll {
             # build is PS job so psake doesn't freak out because it's nested
+            # NOTE: the scriptblock Set-Location handles the working directory;
+            # Start-Job -WorkingDirectory is PS 6+ only and breaks on Windows PowerShell 5.1.
             Start-Job -Scriptblock {
                 Set-Location -Path $using:testModuleSource
                 $global:PSBuildCompile = $false
                 ./build.ps1 -Task Build
-            } -WorkingDirectory $script:testModuleSource | Wait-Job
+            } | Wait-Job
             Write-Debug "TestModule output path: $script:testModuleSource"
             $items = Get-ChildItem -Path $script:testModuleSource -Recurse -File
             Write-Debug ($items | Format-Table FullName | Out-String)