ci: disable GHA cache for buildkit

[aluzzardi]

Kept hitting this error:

 #   10:24AM FTL system | failed to up environment: failed to parse error response 429: {"$id":"1","innerException":null,"message":"Request was blocked due to exceeding usage of resource 'Count' in namespace ''.","typeName":"Microsoft.TeamFoundation.Framework.Server.RequestBlockedException, Microsoft.TeamFoundation.Framework.Server","typeKey":"RequestBlockedException","errorCode":0,"eventId":3000}: invalid character 'ï' looking for beginning of value

See moby/buildkit#2276

/cc @samalba @crazy-max