Fix trailing slash in VFOX_NODEJS_MIRROR causing double slashes in URL paths

[Copilot]

Setting VFOX_NODEJS_MIRROR=https://nodejs.org/dist/ (with trailing slash) produced invalid URLs like https://nodejs.org/dist//v22.18.0/.

Changes

• lib/util.lua: Strip trailing slash from mirror URL in getBaseUrl() using gsub("/$", "")
• README.md: Document that trailing slashes are automatically handled

-- Before
return mirror

-- After
return mirror:gsub("/$", "")

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

VFOX_NODEJS_MIRROR https://nodejs.org/dist/ 会导致拼出来的路径无法请求：
https://nodejs.org/dist//v22.18.0/

记得把文档什么的都更新一下

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