[vcpkg] Vcpkg appinstall deps fixes

[sandercox]

@Shatur95 mentioned in issue #1653 that he was missing _install if install is not overriden with a custom function than _install will not be found so we should refer to install in this function

Also I was prefixing the ${CMAKE_INSTALL_PREFIX} in my custom install override but as @Shatur95 points out it makes more sense to do that in the vcpkg.cmake file.

This is a continuation/bugfix on #13011

[Shatur]

Thank you, it works for me even with CPack.

Maybe introduce an option to override install command as it was made originally?

[BillyONeal]

Thanks for the review @ras0219

[BillyONeal]

@sandercox Thanks for your contribution!