Get exit code of 0 with --only-downloads option

[klalumiere]

Without this fix

#697 changed vcpkg's behavior.

It used to be possible to call $VCPKG_ROOT/vcpkg install --only-downloads and get an exit code of 0. This could be useful to, for instance, download dependencies' sources in order to scan them with Snyk.

After #697, the exit code is now 1, and the GitHub Workflow fails before the Snyk scan.

With this fix

I do not change the behavior of --keep-going, but I re-establish the original behavior of --only-downloads. Hence, $VCPKG_ROOT/vcpkg install --only-downloads succeeds with the exit code 0.

I tested that the fix behaves as expected manually, making sure that I get an exit code of 1 without the fix and an exit code of 0 with the fix.

c.c. @ras0219-msft from #697 and @BillyONeal from our discussion on Discord.

[ras0219-msft]

Thanks for the PR! I think the direction of this change is broadly good (--only-downloads clearly shouldn't consider "I only downloaded" as a failure) -- though I also want to suggest the workaround of ./vcpkg || true

[klalumiere]

Yeah, I thought about ./vcpkg || true, but it would be a bit problematic, so I see it as a last resort (that would have to come with a bit of hacking).

The problem is that there is some good reasons for vcpkg install --only-downloads to fail, and we sometimes encounter some of these failures. For instance, people sometimes use overrides that are not available within the context in which we run vcpkg install --only-downloads. Hence, we get a failure in those cases, which is a good thing. I'm confident that there are other reasons why the command could fail that I can't imagine right now.

If I were to use the last resort || true, I guess I would add a script that would do a check along the lines of is there at least 100 subfolders in the folder buildtrees. That would be in order to know that the only-downloads step worked. This is not ideal (it doesn't really check if the only-download step worked).

[BillyONeal]

Thanks!