fbt: clear PYTHONPATH in env setup (#1585)

scripts/toolchain/fbtenv.cmd

@@ -31,6 +31,7 @@ if not "%REAL_TOOLCHAIN_VERSION%" == "%FLIPPER_TOOLCHAIN_VERSION%" (
 
 set "HOME=%USERPROFILE%"
 set "PYTHONHOME=%FBT_TOOLCHAIN_ROOT%\python"
+set "PYTHONPATH="
 set "PATH=%FBT_TOOLCHAIN_ROOT%\python;%FBT_TOOLCHAIN_ROOT%\bin;%FBT_TOOLCHAIN_ROOT%\protoc\bin;%FBT_TOOLCHAIN_ROOT%\openocd\bin;%PATH%"
 set "PROMPT=(fbt) %PROMPT%"