BUGFIX: Fail if no 'os' is specified in build profile and want to run an exte…

[lasote]

Changelog: Bugfix: If the 'os' is not specified in the build profile and a recipe, in Windows, wanted to run a command, it failed to prepend the build environment launcher causing a hard-to-debug command line error. Now Conan raises if the "os" is not declared in that case. Docs: omit

Close https://github.com/conan-io/conan/issues/11727