[vcpkg] use VCPKG_ROOT_DIR instead of DOWNLOADS for WORKING_DIRECTORY in do_version_check

[lebdron]

Add check for downloads directory existence in do_version_check to prevent No such file or directory error in execute_process.

[BillyONeal]

The correct response to "the downloads directory doesn't exist" is "create the downloads directory", not "dump the files in some random place wherever". Do you have a repro for this issue?

[lebdron]

Yes. On arm64-osx, I install all the required tools - cmake and ninja, prior to any operations, run bootstrap with useSystemBinaries and allowAppleClang, then try to build some packages, and when it tries to test the version of system-installed ninja, it fails on this step, as nothing was downloaded, and downloads directory does not exist.

[BillyONeal]

I see, the repro is arm64-osx.

To be clear, I'm not saying "it's impossible for this to happen", I'm saying "this is not a correct fix for the problem".

[lebdron]

I reproduced this on x64-linux as well. The thing is that it silently fails:

[DEBUG] -- Found ninja('') but at least version 1.10.1 is required! Trying to use internal version if possible!                                                                                                     
[DEBUG] -- Downloading https://github.com/ninja-build/ninja/releases/download/v1.10.1/ninja-linux.zip...

But because it downloads the binary for correct architecture, further build does not fail. This is not the case with arm64, however.

[lebdron]

I also think that "dump the files in some random place wherever" is not the exact description of the issue. do_version_check simply runs /usr/bin/ninja --version, for example, and the directory it is running this command from does not seem to be a big issue.

[BillyONeal]

The presence of the existing working directory setting strongly indicate that it was relevant for at least one program.

[Neumann-A]

There is a high probability that the vcpkg_execute_in_download_mode command was just copied from msiexec call and having the call be inside a vcpkg dir seemed to be safe. Using VCPKG_ROOT or BUILDTREE_DIR seems also be reasonable

[lebdron]

Let it be VCPKG_ROOT then

[lebdron]

Ping @BillyONeal . Are you sure that working directory is required at all? CMake uses absolute paths for executables here.

[lebdron]

I would also ping @Neumann-A , as the macro was introduced here #12895 and not changed much since then.

[BillyONeal]

I did some testing to make sure this didn't edit anything in the root dir.