Add mirrors for downloading lua

[estebanfer]

There were a few times the official lua download website was down, causing CI to fail. Adding a few mirrors by default in case the server is down should prevent those issues. There are a few mirrors that would work:

• https://web.archive.org/web/202if_/https://www.lua.org/ftp/lua-${LUA_VANILLA_VERSION}.tar.gz (202 means latest available archived site in year 202X, if_ makes it to return the exact same content the site returned, not including web archive added stuff)
• https://www.tecgraf.puc-rio.br/lua/mirror/ftp/lua-${LUA_VANILLA_VERSION}.tar.gz

I suppose the same web archive approach could be applied to luajit and others

Adding more URLs to FetchContent_Declare makes it to use the next ones in case the first ones don't work, like

FetchContent_Declare(
    lua-vanilla
    URL ${LUA_VANILLA_DOWNLOAD_URL} ${LUA_VANILLA_DOWNLOAD_URL_MIRROR}
)

${LUA_VANILLA_DOWNLOAD_URL} will be tried first, if it fails, it will try ${LUA_VANILLA_DOWNLOAD_URL_MIRROR} after