Fix CI failure: detect Lua from LUA env var when pkg-config is unavailable

[cclauss]

Fixes the failing GitHub Actions job ci (ubuntu-26.04, 5.1, leafo).

Root Cause

The leafo/gh-actions-lua action installs Lua into a local .lua/ directory without generating a pkg-config .pc file. setup.py relied solely on pkg-config --exists lua5.x to locate Lua, which silently failed for all versions, raising:

Exception: No compatible version of Lua found. Tried ['5.5', '5.4', '5.3', '5.2', '5.1']

Fix

Modified setup.py to fall back to the LUA environment variable (set by leafo/gh-actions-lua to the path of the Lua binary) when pkg-config finds no compatible Lua version:

• Runs lua -v on the binary to parse the version number via regex.
• Derives LUA_INCDIR and LUA_LIBDIR from the binary's parent directory (e.g. .lua/include/ and .lua/lib/).
• Builds the C extension using these manually specified paths and the lua library name (as produced by a source build), bypassing pkg-config for Lua only.

The existing pkg-config-based path is unchanged, so system-installed Lua installations continue to work as before.