Switch xonsh source from PyPI to GitHub tag, add Woodpecker CI

Use fetchFromGitHub with tag = version so the version string and
source are inherently linked via rec. Adds .woodpecker.yaml to
build all flake packages on tag push.

flake.nix

@@ -31,8 +31,7 @@
         hello-world = pkgs.callPackage ./packages/hello-world.nix { };
         xonsh = pkgs.callPackage ./packages/xonsh.nix {
           xonsh-unwrapped = import ./packages/xonsh-unwrapped.nix {
-            inherit (pkgs) lib;
-            inherit (pkgs) python3Packages;
+            inherit (pkgs) lib python3Packages fetchFromGitHub;
           };
         };
       };