ci: win: enable powershell's strict mode

Give variables a default value to pass strict mode.
$ErrorActionPreference defines the default behavior
if a powershell command fails.
If it's set to 'stop', then it aborts the script
on the first unresolved error.
This behavior extends to native programs like cmake.

ci/build.ps1

@@ -1,4 +1,5 @@
-Set-PSDebug -Trace 1
+$ErrorActionPreference = 'stop'
+Set-PSDebug -Strict -Trace 1
 
 $env:CONFIGURATION -match '^(?<compiler>\w+)_(?<bits>32|64)(?:-(?<option>\w+))?$'
 $compiler = $Matches.compiler
@@ -12,6 +13,7 @@ $nvimCmakeVars = @{
   CMAKE_BUILD_TYPE = $cmakeBuildType;
   BUSTED_OUTPUT_TYPE = 'nvim';
 }
+$uploadToCodeCov = $false
 
 # For pull requests, skip some build configurations to save time.
 if ($env:APPVEYOR_PULL_REQUEST_HEAD_COMMIT -and $env:CONFIGURATION -match '^(MSVC_64|MINGW_32|MINGW_64-gcov)$') {
@@ -20,6 +22,7 @@ if ($env:APPVEYOR_PULL_REQUEST_HEAD_COMMIT -and $env:CONFIGURATION -match '^(MSV
 
 function exitIfFailed() {
   if ($LastExitCode -ne 0) {
+    Set-PSDebug -Off
     exit $LastExitCode
   }
 }
@@ -104,10 +107,10 @@ Set-PSDebug -Off
 cmake --build . --config $cmakeBuildType --target functionaltest -- $cmakeGeneratorArgs 2>&1 |
   foreach { $failed = $failed -or
     $_ -match 'functional tests failed with error'; $_ }
-Set-PSDebug -Trace 1
 if ($failed) {
   exit $LastExitCode
 }
+Set-PSDebug -Strict -Trace 1
 
 
 if ($uploadToCodecov) {