Fix vcvars calls. Fix MSVC DLL build step echos

[iboB]

This is related to #3601

Note that the CI is expected to fail after this commit. The fix is in the aforementioned PR

Two CI steps weren't being run. Calling vcvarsall causes the CI step to immediately complete with a success ignoring subsequent commands.

The fix is to use call of a complete batch file (the steps are being exectuted as a batch file and the correct way to "source" batch from batch is to use call)

Another fix here is the cmd echo-s in the MSVC DLL build step. echo on cmd doesn't work the same way it does on bash. The single quotes are literally echoed back. So removing the quotes fixes echo to output valid C++

[iboB]

Expected error which is fixed by #3601 is here: https://github.com/ocornut/imgui/pull/3603/checks?check_run_id=1420415264#step:9:27

[ocornut]

Thanks a lot for investigating this!
It's not everyday we intentionally and happily merge a PR that breaks CI :P

[ocornut]

Merged jointly in fcc2b71, thank you!