util/disk_cache: Use os independent functions instead of getenv

In preparation to allow disk cache to work on Android.

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36821>

src/util/disk_cache_os.h

@@ -127,7 +127,7 @@ struct disk_cache_put_job {
    struct cache_item_metadata cache_item_metadata;
 };
 
-char *
+const char *
 disk_cache_generate_cache_dir(void *mem_ctx, const char *gpu_name,
                               const char *driver_id,
                               const char *cache_dir_name_custom,
@@ -167,8 +167,7 @@ void
 disk_cache_touch_cache_user_marker(char *path);
 
 bool
-disk_cache_mmap_cache_index(void *mem_ctx, struct disk_cache *cache,
-                            char *path);
+disk_cache_mmap_cache_index(void *mem_ctx, struct disk_cache *cache);
 
 void
 disk_cache_destroy_mmap(struct disk_cache *cache);