ADD emsdk env vars to VCPKG_KEEP_ENV_VARS (#23997)
### Description
The vars are set by cmake\external\emsdk\emsdk_env.bat
### Motivation and Context
By default they are filtered by vcpkg to make build reproducible.
However, emscripten's cmake toolchain file needs this information.
emcc.bat has the following code:
```
@set EM_PY=%EMSDK_PYTHON%
@if "%EM_PY%"=="" (
set EM_PY=python
)
```
Actually, it doesn't work as expected. the line
```
set EM_PY=python
```
should be changed to
```
set EM_PY=python.exe
```
We haven't hit this issue because usually the var EM_PY is set.
