Add return type of findAvailableLogFile (#5189)

Co-authored-by: Bryan Kendall <bkend@google.com>

src/bin/firebase.ts

@@ -38,7 +38,7 @@ import * as winston from "winston";
 let args = process.argv.slice(2);
 let cmd: Command;
 
-function findAvailableLogFile() {
+function findAvailableLogFile(): string {
   const candidates = ["firebase-debug.log"];
   for (let i = 1; i < 10; i++) {
     candidates.push(`firebase-debug.${i}.log`);