build: drop the pkg-config requirement from the Windows build (#15469)

The FFmpeg camera view port made pkg-config a required build tool on
Windows. Windows does not ship one, so every Windows developer has to
install it before the build will configure:

  Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
  Call Stack (most recent call first):
    CMakeLists.txt:480 (find_package)

Nothing on Windows needs it. FFmpeg there is a prebuilt zip unpacked
into the deps prefix, whose DLLs the top level CMakeLists already names
by exact soname. The version is fixed before configure runs, so
find_library against that prefix does the job, as on macOS.

Also drops the CI step that installed pkg-config, gated on !SELF_HOSTED
so it never ran on self-hosted runners, and re-comments the if(WIN32)
block that #15234 uncommented only for that find_package.

CMakeLists.txt

@@ -476,8 +476,7 @@ set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 # WIN10SDK_PATH is used to point CMake to the WIN10 SDK installation directory.
 # We pick it from environment if it is not defined in another way
 # ORCA: Removed Netfabb STL fixing service support in favor of CGAL.
-if(WIN32)
-    find_package(PkgConfig REQUIRED)
+# if(WIN32)
 #     if(NOT DEFINED WIN10SDK_PATH)
 #         if(DEFINED ENV{WIN10SDK_PATH})
 #                 set(WIN10SDK_PATH "$ENV{WIN10SDK_PATH}")
@@ -513,7 +512,7 @@ if(WIN32)
 #     else()
 #         message("Building without Win10 Netfabb STL fixing service support")
 #     endif()
-endif()
+# endif()
 
 if (APPLE)
     message("OS X SDK Path: ${CMAKE_OSX_SYSROOT}")