[vcpkg.cmake] add VCPKG_INSTALLED_DIR as a settable option (#16303)

this change does not actually make any changes,
except that `VCPKG_INSTALLED_DIR` and sets `_VCPKG_INSTALLED_DIR`.
In the future, we can start using `VCPKG_INSTALLED_DIR`
completely.

scripts/buildsystems/vcpkg.cmake

@@ -362,16 +362,22 @@ if(NOT Z_VCPKG_ROOT_DIR)
     z_vcpkg_add_fatal_error("Could not find .vcpkg-root")
 endif()
 
-if(NOT DEFINED _VCPKG_INSTALLED_DIR)
-    if(VCPKG_MANIFEST_MODE)
-        set(_VCPKG_INSTALLED_DIR "${CMAKE_BINARY_DIR}/vcpkg_installed")
-    else()
-        set(_VCPKG_INSTALLED_DIR "${Z_VCPKG_ROOT_DIR}/installed")
-    endif()
-set(_VCPKG_INSTALLED_DIR "${_VCPKG_INSTALLED_DIR}"
+if(DEFINED VCPKG_INSTALLED_DIR)
+    # do nothing
+elseif(DEFINED _VCPKG_INSTALLED_DIR)
+    set(VCPKG_INSTALLED_DIR "${_VCPKG_INSTALLED_DIR}")
+elseif(VCPKG_MANIFEST_MODE)
+    set(VCPKG_INSTALLED_DIR "${CMAKE_BINARY_DIR}/vcpkg_installed")
+else()
+    set(VCPKG_INSTALLED_DIR "${Z_VCPKG_ROOT_DIR}/installed")
+endif()
+
+set(VCPKG_INSTALLED_DIR "${VCPKG_INSTALLED_DIR}"
+    CACHE PATH
+    "The directory which contains the installed libraries for each triplet" FORCE)
+set(_VCPKG_INSTALLED_DIR "${VCPKG_INSTALLED_DIR}"
     CACHE PATH
     "The directory which contains the installed libraries for each triplet" FORCE)
-endif()
 
 if(CMAKE_BUILD_TYPE MATCHES "^[Dd][Ee][Bb][Uu][Gg]$" OR NOT DEFINED CMAKE_BUILD_TYPE) #Debug build: Put Debug paths before Release paths.
     list(APPEND CMAKE_PREFIX_PATH