Fixed unbound variable error for MODRINTH_EXCLUDE_FILES (#2528)

scripts/start-deployModrinth

@@ -10,6 +10,7 @@ resultsFile=/data/.install-modrinth.env
 : "${MODRINTH_LOADER:=}"
 : "${MODRINTH_VERSION:=${MODRINTH_VERSION_ID:-}}"
 : "${MODRINTH_IGNORE_MISSING_FILES:=}"
+: "${MODRINTH_EXCLUDE_FILES:=}"
 
 if [[ ! $MODRINTH_MODPACK ]]; then
   log "ERROR: MODRINTH_MODPACK must be set when using TYPE/MOD_PLATFORM of MODRINTH"