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
but do not depend on it.

PowerShell/PowerShell#3996

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
   }
 }
@@ -72,18 +75,20 @@ python  -c "import neovim; print(str(neovim))" ; exitIfFailed
 python3 -c "import neovim; print(str(neovim))" ; exitIfFailed
 
 $env:PATH = "C:\Ruby24\bin;$env:PATH"
-cmd /c gem.cmd install neovim ; exitIfFailed
-where.exe neovim-ruby-host.bat ; exitIfFailed
+gem.cmd install neovim
+Get-Command -CommandType Application neovim-ruby-host.bat
 
-cmd /c npm.cmd install -g neovim ; exitIfFailed
-where.exe neovim-node-host.cmd ; exitIfFailed
-cmd /c npm link neovim
+npm.cmd install -g neovim
+Get-Command -CommandType Application neovim-node-host.cmd
+npm.cmd link neovim
 
 function convertToCmakeArgs($vars) {
   return $vars.GetEnumerator() | foreach { "-D$($_.Key)=$($_.Value)" }
 }
 
-mkdir .deps
+if (-Not (Test-Path -PathType container .deps)) {
+  mkdir .deps
+}
 cd .deps
 cmake -G $cmakeGenerator $(convertToCmakeArgs($depsCmakeVars)) ..\third-party\ ; exitIfFailed
 cmake --build . --config $cmakeBuildType -- $cmakeGeneratorArgs ; exitIfFailed
@@ -104,10 +109,10 @@ 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) {