make building the manuals optional

since we currently can't fetch the manuals as a tarball from Hydra, we
have to evaluate all the Nix releases under the sun, which takes quite
a while. most of the time this is not needed and is in the way to work
on the site, so disable it by default.

.github/workflows/build-and-deploy.yml

@@ -20,7 +20,7 @@ jobs:
 
     - name: Build
       run: |
-        nix-build
+        nix-build --arg withManuals true
 
         # The netlify action doesn't follow symlinks properly.
         mkdir ./dist

CONTRIBUTING.md

@@ -27,6 +27,16 @@ To manually test [redirects](./_redirects):
 [nix-shell:nix.dev]$ netlify dev -d result
 ```
 
+## Building the reference manuals
+
+By default nix.dev builds without the various versions of the Nix reference manual, as that takes quite a while due to how it's currently implemented.
+To enable building the manuals:
+
+```shell-session
+[nix-shell:nix.dev]$ nix-build -A build --arg withManuals true
+[nix-shell:nix.dev]$ devmode --arg withManuals true
+```
+
 ## Updating reference manuals
 
 With the current setup, the [Nix manual hosted on nix.dev](https://nix.dev/reference/nix-manual) does not get updated automatically with new releases.

default.nix

@@ -6,6 +6,7 @@
     overlays = [ (import ./nix/overlay.nix) ];
     inherit system;
   },
+  withManuals ? false, # building the manuals is expensive
 }:
 let
   lib = pkgs.lib;
@@ -34,7 +35,7 @@ let
           };
         in
         ''
-          cp -f ${substitutedNixManualReference} source/reference/nix-manual.md
+          ${lib.optionalString withManuals "cp -f ${substitutedNixManualReference} source/reference/nix-manual.md"}
           make html
         '';
       installPhase =
@@ -62,6 +63,7 @@ let
         ''
           mkdir -p $out/manual/nix
           cp -R build/html/* $out/
+        '' + lib.optionalString withManuals ''
           ${lib.concatStringsSep "\n" (lib.mapAttrsToList release releases.nixReleases)}
           ${lib.concatStringsSep "\n" (lib.mapAttrsToList mutableRedirect releases.mutableNixManualRedirects)}
         '';
@@ -76,12 +78,13 @@ let
         from livereload import Server
         from subprocess import Popen, PIPE
         import shlex
+        import sys
 
         server = Server()
 
         def nix_build():
           p = Popen(
-            shlex.split("nix-build -A build"),
+            shlex.split("nix-build -A build") + sys.argv[1:],
             # capture output as text
             stdout=PIPE,
             stderr=PIPE,