Install ffmpeg from vcpkg in gazebo builds.

[nuclearsandwich]

This dependency was previously optional and seemingly omitted from Windows build farm builds. However it is now required based empirically on recently failing jobs.

Since ffmpeg is not a dependency that we've built and distributed via s3 we can try to install the version from vcpkg for now.

[scpeters]

I assume this is in response to failures such as the following which fails due to a lack of the av component of ign-common / gz-common:

• https://build.osrfoundation.org/view/ign-garden/job/ign_gazebo-ci-win/185/

I believe ffmpeg is actually installed on these machines and that the problem is actually a failure to find the dependencies. See gazebosim/gz-common#386 for more info.

[scpeters]

see gazebosim/gz-cmake#271

[Blast545]

FYI: @Crola1702

[scpeters]

gz-sim windows builds are now unstable: gazebosim/gz-sim#1578

[Blast545]

I see that the PR jobs were yellow with the PR merged, but now main is seems to be showing an error earlier in the stack, where pkg-config information for Ign-Ogre is not found. Are you aware / tracking this? https://build.osrfoundation.org/job/ign_gazebo-ci-win/208/consoleFull

Although that's a different error, I won't close this one yet as the new error is happening earlier in the stack.

[scpeters]

I see that the PR jobs were yellow with the PR merged, but now main is seems to be showing an error earlier in the stack, where pkg-config information for Ign-Ogre is not found. Are you aware / tracking this? https://build.osrfoundation.org/job/ign_gazebo-ci-win/208/consoleFull

Although that's a different error, I won't close this one yet as the new error is happening earlier in the stack.

the Ing-Ogre message is just a warning; here is the error:

CMake Error at C:/Program Files/CMake/share/cmake-3.20/Modules/FindBoost.cmake:1793 (file):
  file STRINGS file
  "C:/vcpkg/installed/x64-windows/include/boost/version.hpp" cannot be read.
Call Stack (most recent call first):
  C:/vcpkg/scripts/buildsystems/vcpkg.cmake:782 (_find_package)
  CMakeLists.txt:98 (find_package)

I'm not tracking this as I've used up my maintenance time for this week

[Blast545]

The job is not failing anymore, so this PR can be closed. Steven! is OoO, as we opened this PR together, I'll take the freedom to close it. Thanks!