Updating the mmctl process to set the value once and than use it. Also added the basic auth code from agniva

Makefile

@@ -115,7 +115,7 @@ ALL_PACKAGES=$(TE_PACKAGES)
 endif
 
 # Decide what version of prebuilt binaries to download. This will use the release-* branch names or change to the latest.
-MMCTL_REL_TO_DOWNLOAD = $(shell scripts/get_latest_release.sh 'mattermost/mmctl' 'release-')
+MMCTL_REL_TO_DOWNLOAD:=$(shell scripts/get_latest_release.sh 'mattermost/mmctl' 'release-')
 
 all: run ## Alias for 'run'.