SL-20546: Allow running build.sh even without GITHUB_OUTPUT set.

Default it to /dev/null, so output to $GITHUB_OUTPUT is permitted but
discarded.

build.sh

@@ -112,7 +112,8 @@ installer_CYGWIN()
   fi
 }
 
-[[ -n "$GITHUB_OUTPUT" ]] || fatal "Need to export GITHUB_OUTPUT"
+# if someone wants to run build.sh outside the GitHub environment
+[[ -n "$GITHUB_OUTPUT" ]] || export GITHUB_OUTPUT='/dev/null'
 # The following is based on the Warning for GitHub multiline output strings:
 # https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#multiline-strings
 EOF=$(dd if=/dev/urandom bs=15 count=1 status=none | base64)