ci : update + packages (#146)

* Add simple Debian package

* Update CI configuration

* Add cpack Deb package for libggwave-dev

* Add `make deb` top level target and CI job

* Ensure `make deb` packages are deterministic

Update CMakeLists.txt to make cpack generated Debian packages
deterministic build outputs by:
- Setting SOURCE_DATE_EPOCH to 0
- Executing `git log -1 --pretty=%ct` and storing the result in
  SOURCE_DATE_EPOCH, overriding 0
- Export SOURCE_DATE_EPOCH to the environment
- Explicitly prefix the execution of the cpack command in the custom
  target `deb_c_library` with SOURCE_DATE_EPOCH

Update `README.md-tmpl.md` and `README.md` to document new `make deb`
target.

The Makefile for the Python bindings already handle deterministic
building with the same strategy.

* libc6, not libc

bindings/python/debian/copyright

@@ -0,0 +1,14 @@
+Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Upstream-Name: ggwave
+Upstream-Contact: Georgi Gerganov <ggerganov@gmail.com>
+Source: https://github.com/ggerganov/ggwave
+
+Files: *
+Copyright: Georgi Gerganov <ggerganov@gmail.com>
+License: MIT
+
+Files: debian/*
+Copyright: Georgi Gerganov <ggerganov@gmail.com>
+License: MIT
+
+License: MIT