Updates latest-dagger-version.sh script.

This CL updates `latest-dagger-version.sh` so that it can be easily run outside of GitHub Actions without a token.

RELNOTES=N/A
PiperOrigin-RevId: 528004661

util/latest-dagger-version.sh

@@ -4,15 +4,16 @@ set -eu
 
 function github-rest-api {
   local GITHUB_REST_API=$1
-
   local GITHUB_API_HEADER_ACCEPT="Accept: application/vnd.github.v3+json"
+  # Grab the GH_TOKEN or else default to an empty string.
+  local GITHUB_TOKEN="${GH_TOKEN:-}"
 
-  if [ -z "$GH_TOKEN" ]; then
+  if [ -z "$GITHUB_TOKEN" ]; then
     curl -s $GITHUB_REST_API -H $GITHUB_API_HEADER_ACCEPT
   else
     curl -s $GITHUB_REST_API \
       -H $GITHUB_API_HEADER_ACCEPT \
-      -H "authorization: Bearer $GH_TOKEN"
+      -H "authorization: Bearer $GITHUB_TOKEN"
   fi
 }