used fixed block caching for some intergation tests

Signed-off-by: Brian L. Troutwine <brian.troutwine@datadoghq.com>

integration/sheepdog/src/lib.rs

@@ -241,6 +241,7 @@ generator:
           post:
             maximum_prebuild_cache_size_bytes: "8 Mb"
             variant: "apache_common"
+            block_cache_method: Fixed
         "#,
         )?;
 
@@ -276,6 +277,7 @@ generator:
           post:
             maximum_prebuild_cache_size_bytes: "8 Mb"
             variant: "ascii"
+            block_cache_method: Fixed
         "#,
         )?;