PCbuild/build.bat: Add note about using msbuild response file. (#1551)

Using a response file will eliminate the headache associated with batch argument/quote processing.

For example I unsucessfully compiled python with visualcpptools when specifying VSInstallDir in the batch file (cannot find vcruntime.h)

```batch
build.bat -p x64 -e -M --no-tkinter  "/p:VCInstallDir=%VCInstallDir%"
```

but it build successfully when specifying it in a response file

msbuild.rsp:
```
/p:VCInstallDir=%VCInstallDir%
```