Meta: Promote SERENITY_CACHE_DIR to a real option

We've had it as a secret option for long enough, let's make it more
visible.

Meta/Azure/Lagom.yml

@@ -51,7 +51,7 @@ jobs:
         os: '${{ parameters.os }}'
         arch: 'Lagom'
         toolchain: '$(toolchain)'
-        download_cache_path: 'Meta/Lagom/Build'
+        download_cache_path: 'Meta/Lagom/caches'
         serenity_ccache_path: '$(SERENITY_CCACHE_DIR)'
         with_remote_data_caches: true
         ${{ if eq(parameters.os, 'macOS') }}:

Meta/CMake/Superbuild/CMakeLists.txt

@@ -44,9 +44,6 @@ if(NOT SERENITY_TOOLCHAIN STREQUAL "GNU")
 endif()
 set(SERENITY_BUILD_DIR "${PROJECT_BINARY_DIR}/../${SERENITY_ARCH}${SERENITY_BUILD_DIR_SUFFIX}")
 
-# Location to cache artifacts downloaded during the build.
-file(REAL_PATH "${PROJECT_BINARY_DIR}/../caches" SERENITY_CACHE_DIR)
-
 # Pkgconf incorrectly discards a sysroot if it doesn't match the start of the path to the
 # library file. To avoid that, resolve our sysroot into an absolute and canonical path
 # that matches pkgconf's result for resolving the library file.

Meta/CMake/common_options.cmake

@@ -24,3 +24,5 @@ serenity_option(HACKSTUDIO_BUILD OFF CACHE BOOL "Automatically enabled when buil
 
 serenity_option(ENABLE_JAKT OFF CACHE BOOL "Enable building jakt files")
 serenity_option(JAKT_SOURCE_DIR "" CACHE STRING "Pre-existing jakt language source directory")
+
+serenity_option(SERENITY_CACHE_DIR "${PROJECT_BINARY_DIR}/../caches" CACHE PATH "Location of shared cache of downloaded files")

Meta/Lagom/CMakeLists.txt

@@ -25,8 +25,6 @@ get_filename_component(
 )
 set(SerenityOS_SOURCE_DIR "${SERENITY_PROJECT_ROOT}" CACHE STRING "")
 
-set(SERENITY_CACHE_DIR "${PROJECT_BINARY_DIR}" CACHE STRING "")
-
 list(APPEND CMAKE_MODULE_PATH "${SERENITY_PROJECT_ROOT}/Meta/CMake")
 
 if(NOT COMMAND serenity_option)