Fix initializing BuildTools when the Platform environment variable is…

… set (dotnet/coreclr#24998)

* Make build-test.cmd exit if init-tools.cmd fails

* Clear Platform variable before initializing BuildTools


Commit migrated from dotnet/coreclr@a0486f5

src/coreclr/build-test.cmd

@@ -166,6 +166,9 @@ REM ===
 REM =========================================================================================
 
 call "%__ProjectDir%\init-tools.cmd"
+if NOT [%ERRORLEVEL%]==[0] (
+    exit /b %ERRORLEVEL%
+)
 @if defined _echo @echo on
 
 set "__ToolsDir=%__ProjectDir%\Tools"

src/coreclr/init-tools.cmd

@@ -1,6 +1,10 @@
 @if not defined _echo @echo off
 setlocal
 
+:: Clear the 'Platform' environment variable for this session
+:: This avoids overriding the default value from BuildTools projects used for initialization
+set Platform=
+
 set INIT_TOOLS_LOG=%~dp0init-tools.log
 if [%PACKAGES_DIR%]==[] set PACKAGES_DIR=%~dp0.packages
 if [%TOOLRUNTIME_DIR%]==[] set TOOLRUNTIME_DIR=%~dp0Tools