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'.
 

scripts/get_latest_release.sh

@@ -12,7 +12,7 @@ BASIC_AUTH=""
 # If we find a github username and token, we use that.
 # In CI, these variables are available and useful to avoid rate limits which is
 # much more strict for unauthenticated requests.
-if [[ $GITHUB_USERNAME != "" && $GITHUB_TOKEN != "" ]];
+if [[ ! -z "$GITHUB_USERNAME" && ! -z "$GITHUB_TOKEN" ]];
 then
 	BASIC_AUTH="--user $GITHUB_USERNAME:$GITHUB_TOKEN"
 fi