docs(readme): add tip to configure luarocks install with envvars

[pysan3]

When lua and luajit is not installed via the system package manager, luarocks build script
fails to detect the lua runtime, which results in a fail.

Usually, we can ./configure with options such as --with-lua (cli options), but as this is run in the background, I cannot edit these options, or I would need to add a new field in the UI which is kinda hassle.

Luarocks build scripts uses variables such as LUA_DIR, so we can trick the script by providing these varibles from outside.
To workaround this issue, I added a [!TIP] in the README for other people who came across the same issue.

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Tip

In order to configure luarocks installation to use a specific lua install,
use environment variables LUA_BINDIR=<Directory of lua binary> and LUA_BINDIR_SET=yes.

For example:

LUA_BINDIR="${XDG_BIN_DIR:-$HOME/.local/bin}" LUA_BINDIR_SET=yes nvim -u NORC -c "source ...

[github-actions]

Review Checklist

Does this PR follow the Contribution Guidelines? Following is a partial checklist:

Proper conventional commit scoping:

• For example, fix(installer): some installer bugfix

• Pull request title has the appropriate conventional commit prefix.

If applicable:

• Tested
  • Tests have been added.
  • Tested manually (steps in PR description).
• Updated documentation.

[mrcjkb]

Thanks again 😄