Use vcpkg version 2021.05.12 for CI

[DomClark]

The LV2 port is broken in the latest master version of vcpkg. I have submitted a pull request to fix this (microsoft/vcpkg#20102), but until that is merged, this PR should fix our MSVC builds by using an older version of vcpkg. I have chosen the most recently tagged version.

Changes:

• Pull 2021.05.12 tag instead of master branch for vcpkg on AppVeyor.
• Pass --quiet flag to git pull - currently it generates over 16000 lines of useless noise in our build logs.
• Add .appveyor.yml as a dependency for the vcpkg port cache. This is primarily to force a rebuild of the LV2 port with this PR, but may be useful for modifying ports in the future.

In the future, we may want to consider using a manifest file for vcpkg, which would allow ports and their versions to be specified outside of the CI config script, and provide a consistent set of ports for both CI and local builds. Currently, however, manifest files are in beta and port versioning is still experimental and hidden behind a flag, so I am avoiding that for now.

[DomClark]

Closing since the vcpkg PR was merged.

[PhysSong]

Why don't you reopen this and merge other changes?

[DomClark]

I think the git pull --quiet change is still useful - I'll commit that straight to master. The others are less so:

• Obviously we don't need to pull an older version any more, and I'm hoping that if something like this happens again, port versioning will be stable enough that we can use it instead.
• The cache dependency doesn't seem to be required any more - the old cache seems to be gone, and AppVeyor won't cache anything new until the build is successful. As such, I think we should leave this until we need it. Ideally we would use a manifest file and make the port cache depend on that - a dependency on the entire AppVeyor configuration is a bit excessive. Manifest files, while in beta, are supposed to be stable under certain use cases, so I might experiment with them and open another PR later.