Update CI status badge from Travis to GitHub Actions

[rootkiller6788]

Problem

The README CI badge points to https://travis-ci.org/google/googet.svg. Travis CI
retired its .org service for open-source projects in mid-2021, so the badge is now
stale: it no longer reflects a real build status and links to a dead service.

Change

Replace the Travis badge with a GitHub Actions badge that targets the repository’s own
workflow (.github/workflows/go.yml):

- [![Build Status](https://travis-ci.org/google/googet.svg?branch=master)](https://travis-ci.org/google/googet)
+ [![Go](https://github.com/google/googet/actions/workflows/go.yml/badge.svg)](https://github.com/google/googet/actions/workflows/go.yml)

Why this is safe

• The .github/workflows/go.yml workflow already exists, so the badge has a real target
  and will render as soon as Actions has a run to report.
• Docs-only change: no code, no build, no dependency changes.

Verification

• Confirmed .github/workflows/go.yml is present in the repository.
• Confirmed the new badge URL resolves (HTTP 200).