In lazy.nvim, the `build` parameter accept value in different format.
If the value is
1. a string and starts with `:`, it will be treated as vim command
2. a string and starts without `:`, it will be treated as shell command
3. a function. Lazy.nvim will run this function
I compared closely between case 1 and case 3. In case 1, when lazy.nvim
runs the viml command, the runtimepath is already populated with the
current plugin we are trying to build.
However in case 3, the runtimepath is not populated with the plugin
path, which means that any command you run by this plugin is not
available.
So I did it in a hacky way to add the plugin path to runtimepath
manually, and source it manually using `:runtime` command. The `:
runtime` command is needed because in the init process, adding the
plugin path to runtimepath does not ensure that the script under this
plugin is sourced immediately. So if we want to use the command/function
immediately, we need to source it manually, see also https://github.com/neovim/neovim/issues/29957.
