[MM-18193] Use vendor folder when compiling test plugins (#12188)

build/Jenkinsfile.pr

@@ -416,4 +416,4 @@ pipeline {
 			cleanWs notFailBuild: true
 		}
 	}
-}
+}

build/README.md

@@ -14,6 +14,6 @@ We have a docker image to build `mattermost-server` and it is based on Go docker
 
 In our Docker Hub Repository we have the following images:
 
-- `mattermost/mattermost-build-server:dec-7-2018` which is based on Go 1.11 you can use for MM versions <= `5.9.0`
-- `mattermost/mattermost-build-server:feb-28-2019` which is based on Go 1.12 you can use for MM versions > `5.9.0` <= `5.15.0`
+- `mattermost/mattermost-build-server:dec-7-2018` which is based on Go 1.11 you can use for MM versions <= `5.8.0`
+- `mattermost/mattermost-build-server:feb-28-2019` which is based on Go 1.12 you can use for MM versions >= `5.9.0` <= `5.15.0`
 - `mattermost/mattermost-build-server:sep-17-2019` which is based on Go 1.12.9 you can use for MM versions >= `5.16.0`