Fix nested cmd.exe shell when rerunning init.cmd (#12312)

## Fixes

No linked issue.

## PR Type

- [x] Bugfix
- [ ] Feature
- [ ] Code style update (formatting, renaming)
- [ ] Refactoring (no functional changes, no API changes)
- [ ] Build related changes
- [ ] Documentation content changes
- [ ] Other (please describe):

## Description

### Current Behavior

Rerunning `init.cmd` in an already initialized command window can
unexpectedly enter a nested `cmd.exe` shell, making initialization
appear to stop until the user types `exit`.

`DevCmd.cmd` records `/PreserveContext` using a marker file under
`%TEMP%`. A previous initialization points `%TEMP%` into `BuildOutput`,
which `git clean -xdf` can remove. If the directory is missing, marker
creation fails and `DevCmd.cmd` incorrectly starts `cmd /k`.

### New Behavior

Replace the marker file with an environment flag explicitly carried
across `endlocal`. `/PreserveContext` no longer depends on a writable,
existing temporary directory.

Calls from `init.cmd` return without opening a nested shell. Invoking
`DevCmd.cmd` without `/PreserveContext` retains its interactive-shell
behavior.

## Customer Impact

WinUI developer inner loop tooling only. Prevents interrupted
initialization after cleaning build output. No changes to WinUI runtime
behavior or public APIs.

## Regression Potential

- [x] Low risk — isolated change, limited scope
- [ ] Medium risk — touches shared components or public APIs
- [ ] High risk — architectural or breaking API change

Limited to argument-state handling in `DevCmd.cmd` and so only affects
the WinUI developer inner loop.

## How Has This Been Tested?

- [x] I have performed a self-review of my own code
- [ ] I have added tests to cover my changes
- [ ] Existing tests pass locally

Ran `init`, ran `git clean -xdf`, ran `init`, verified that the nested
`cmd.exe` shell no longer appears.

## Screenshots (if appropriate)

N/A; no visual changes.

Copilot-Session: fc793207-256a-4658-b725-bbc0e551b5ed

DevCmd.cmd

@@ -39,16 +39,16 @@ set PrereleaseArg=
 
 setlocal enableextensions enabledelayedexpansion
 set _ARGS=
+set _PreserveContext=0
 
 rem We are targeting VS 2019 (version 16.x) or later
 set VsVersion=16.0
 
-if exist %temp%\WinUI.PreserveContext.marker del %temp%\WinUI.PreserveContext.marker
 :ParseArgs
 if "%1" EQU "" (
     goto :DoneParsing
 ) else if /i "%1" EQU "/PreserveContext" (
-    echo. > %temp%\WinUI.PreserveContext.marker
+    set _PreserveContext=1
 ) else if /i "%1" EQU "/Prerelease" (
     set PrereleaseArg=-prerelease
 ) else (
@@ -90,6 +90,7 @@ if exist %~dp0.buildtools (
 )
 
 endlocal & (
+    set "_PreserveContext=%_PreserveContext%"
     echo "Initializing VS command prompt from %MSBuildInstallPath%\Common7\Tools\VsDevCmd.bat ..."
     call "%MSBuildInstallPath%\Common7\Tools\VsDevCmd.bat" /no_logo %_ARGS%
 )
@@ -99,11 +100,11 @@ call :SetEnviromentVariable VCToolsInstallDir "%VCToolsInstallDir%"
 call :SetEnviromentVariable VCToolsRedistDir "%VCToolsRedistDir%"
 call :SetEnviromentVariable ExtensionSdkDir "%ExtensionSdkDir%"
 
-if not exist %temp%\WinUI.PreserveContext.marker (
+if not "%_PreserveContext%"=="1" (
     echo Starting cmd /k
     cmd /k "cd /d %RepoRoot%"
 )
 
-if exist %temp%\WinUI.PreserveContext.marker (del %temp%\WinUI.PreserveContext.marker)
+set "_PreserveContext="
 
 exit /b 0