nix: fix nix build and simplify NixOS install docs

CLN's ./configure detects Python through `uv` (see default_python in
configure), which the cln derivation does not provide, so PYTHON ends up
empty and the build fails when a Makefile step runs
`$(PYTHON) devtools/blockreplace.py`. Set PYTHON=python3 in preConfigure
(the derivation already ships a python3 with mako/grpcio-tools) and add
blockreplace.py to patchShebangs.

With `nix build` working again, replace the stale poetry-based NixOS section
of the install guide with the flake commands (nix build / nix run /
nix profile install / nix develop). Verified in a nixos/nix container:
nix build ".?submodules=1" produces lightningd/lightning-cli v26.06.1.

Changelog-Fixed: nix: the flake build (`nix build`) no longer fails because ./configure cannot detect Python.

doc/getting-started/getting-started/installation.md

@@ -317,13 +317,40 @@ Finally, build `c-lightning`:
 
 ## To Build on NixOS
 
-Use nix-shell launch a shell with a full Core Lightning dev environment:
+Core Lightning ships a [Nix flake](https://nixos.wiki/wiki/Flakes), so on NixOS
+you don't need to install any build dependencies by hand. Make sure the
+`nix-command` and `flakes` features are enabled.
+
+Build and run directly from the repository; the git submodules are fetched
+automatically and the binaries are placed under `./result/bin`:
 ```shell
-nix-shell -Q -p gdb sqlite autoconf git clang libtool sqlite autoconf \
-autogen automake gmp zlib gettext libsodium poetry 'python3.withPackages (p: [p.bitcoinlib])' \
-valgrind --run "./configure && poetry shell"
-poetry install
-make
+nix build github:ElementsProject/lightning
+
+# Or run them straight away:
+nix run github:ElementsProject/lightning#lightningd -- --version
+nix run github:ElementsProject/lightning#lightning-cli -- --version
+```
+
+Build a specific release by appending the tag, or install into your profile:
+```shell
+nix build github:ElementsProject/lightning/v26.06.1
+nix profile install github:ElementsProject/lightning
+```
+
+From a local checkout, append `?submodules=1` so the build sees the submodules:
+```shell
+git clone https://github.com/ElementsProject/lightning.git
+cd lightning
+git checkout v26.06.1
+nix build ".?submodules=1"
+./result/bin/lightningd --version
+```
+
+To build with PostgreSQL support use the `cln-postgres` package, and for a
+development shell with the full toolchain use `nix develop`:
+```shell
+nix build ".?submodules=1#cln-postgres"
+nix develop
 ```
 
 ## To Build on macOS Apple Silicon

nix/pkgs/default.nix

@@ -66,7 +66,8 @@ stdenv.mkDerivation {
           tools/update-mocks.sh \
           tools/mockup.sh \
           tools/fromschema.py \
-          devtools/sql-rewrite.py
+          devtools/sql-rewrite.py \
+          devtools/blockreplace.py
       ''
     else
       ''
@@ -76,6 +77,11 @@ stdenv.mkDerivation {
 
   configureFlags = [ "--disable-valgrind" ];
 
+  # ./configure detects Python via `uv` (configure:default_python), which is not
+  # part of this derivation. Point it at the python3 we already provide so the
+  # codegen steps that call $(PYTHON) (e.g. devtools/blockreplace.py) work.
+  preConfigure = "export PYTHON=python3";
+
   enableParallelBuilding = true;
 
   # workaround for build issue, happens only x86_64-darwin, not aarch64-darwin