[MM-28303]: check if mmctl exists before checking on Github and reduce requests to Github (#15387)

Summary:
Every make call would ping Github to find out the mmctl version to download. Each check made 4 requests to Github. So every make execution resulted in 4 requests to Github. This leads to frequent rate-limit errors from Github.
In this PR we check for the mmctl version only if mmctl doesn't already exist. We also print a more helpful error message.
Reduce the number the number of requests to Github from 4 to 2.

Ticket Link:
https://mattermost.atlassian.net/browse/MM-28303

scripts/get_latest_release.sh

@@ -14,39 +14,44 @@ BASIC_AUTH=""
 # much more strict for unauthenticated requests.
 if [[ ! -z "$GITHUB_USERNAME" && ! -z "$GITHUB_TOKEN" ]];
 then
-	BASIC_AUTH="--user $GITHUB_USERNAME:$GITHUB_TOKEN"
+  BASIC_AUTH="--user $GITHUB_USERNAME:$GITHUB_TOKEN"
 fi
 
-LATEST_REL=$(curl \
+LATEST_RELEASE=$(curl \
   --silent \
-	$BASIC_AUTH \
-  "https://api.github.com/repos/$REPO_TO_USE/releases/latest" \
-  | grep '"tag_name":' | sed -E 's/.*"([^"]+)".*/\1/')
+  $BASIC_AUTH \
+  "https://api.github.com/repos/$REPO_TO_USE/releases/latest")
 
-DRAFT=$(curl \
+RELEASES=$(curl \
   --silent \
-	$BASIC_AUTH \
-  "https://api.github.com/repos/$REPO_TO_USE/releases/latest" \
-  | grep '"draft":' | sed -E 's/.*: ([^,]+).*/\1/')
+  $BASIC_AUTH \
+  "https://api.github.com/repos/$REPO_TO_USE/releases")
 
-PRERELEASE=$(curl \
-  --silent \
-	$BASIC_AUTH \
-  "https://api.github.com/repos/$REPO_TO_USE/releases" \
-  | grep '"prerelease":' | sed -E 's/.*: ([^,]+).*/\1/')
+LATEST_REL=$(echo "$LATEST_RELEASE" | grep '"tag_name":' | sed -E 's/.*"([^"]+)".*/\1/')
+
+DRAFT=$(echo "$LATEST_RELEASE" | grep '"draft":' | sed -E 's/.*: ([^,]+).*/\1/')
+
+PRERELEASE=$(echo "$RELEASES" | grep '"prerelease":' | sed -E 's/.*: ([^,]+).*/\1/')
 
 # Check if this is a release branch
 THIS_BRANCH=$(git rev-parse --abbrev-ref HEAD)
 #THIS_BRANCH="release-5.27"# - Used to test release logic on a non release branch
 
 if [[ "$THIS_BRANCH" =~ $BRANCH_TO_USE || $DRAFT =~ "true" ]]; then
-    VERSION_REL=${THIS_BRANCH//$BRANCH_TO_USE/v}
-    REL_TO_USE=$(curl --silent $BASIC_AUTH "https://api.github.com/repos/$REPO_TO_USE/releases" | grep '"tag_name":' | sed -E 's/.*"([^"]+)".*/\1/' | sed -n "/$VERSION_REL/p" | sort -rV | head -n 1)
+  VERSION_REL=${THIS_BRANCH//$BRANCH_TO_USE/v}
+  REL_TO_USE=$(echo "$RELEASES" | grep '"tag_name":' | sed -E 's/.*"([^"]+)".*/\1/' | sed -n "/$VERSION_REL/p" | sort -rV | head -n 1)
 elif [[ "$THIS_BRANCH"  =~ "master" ]]; then
-    # Get the latest release even if its a pre-release
-    REL_TO_USE=$(curl --silent $BASIC_AUTH "https://api.github.com/repos/$REPO_TO_USE/releases" | grep '"tag_name":' | sed -E 's/.*"([^"]+)".*/\1/' | sort -rV | head -n 1)
+  # Get the latest release even if its a pre-release
+  REL_TO_USE=$(echo "$RELEASES" | grep '"tag_name":' | sed -E 's/.*"([^"]+)".*/\1/' | sort -rV | head -n 1)
 else
-    REL_TO_USE=$LATEST_REL
+  REL_TO_USE=$LATEST_REL
 fi
 
-echo "$REL_TO_USE"
+if [[ -z "$REL_TO_USE" ]]
+then
+  echo "An error has occured trying to get the latest mmctl release. Aborting. Perhaps api.github.com is down, or you are being rate-limited.";
+  echo "Set the GITHUB_USERNAME and GITHUB_TOKEN environment variables to the appropriate values to work around Github rate-limiting.";
+  exit 1;
+else
+  echo "$REL_TO_USE"
+fi