Docs build: depend on locally built nix executable and not installed one

Previously many of the documentation targets were depending on
$(bindir)/nix which is the installed version. This meant that its
install rules would be triggered (which in chain would also trigger
the install of libraries, as reported in NixOS#5140). Therefore a build
of the documentation without an installation would not be possible
(which apart from doing unwanted operations it may also generate
permission problems for example).
The fix makes the rules depend on $(nix_DIR)/nix instead, which is
the executable in the build tree.

doc/manual/local.mk

@@ -18,7 +18,7 @@ dummy-env = env -i \
 	NIX_SSL_CERT_FILE=/dummy/no-ca-bundle.crt \
 	NIX_STATE_DIR=/dummy
 
-nix-eval = $(dummy-env) $(bindir)/nix eval --experimental-features nix-command -I nix/corepkgs=corepkgs --store dummy:// --impure --raw
+nix-eval = $(dummy-env) $(nix_DIR)/nix eval --experimental-features nix-command -I nix/corepkgs=corepkgs --store dummy:// --impure --raw
 
 $(d)/%.1: $(d)/src/command-ref/%.md
 	@printf "Title: %s\n\n" "$$(basename $@ .1)" > $^.tmp
@@ -42,31 +42,31 @@ $(d)/src/SUMMARY.md: $(d)/src/SUMMARY.md.in $(d)/src/command-ref/new-cli
 	$(trace-gen) cat doc/manual/src/SUMMARY.md.in | while IFS= read line; do if [[ $$line = @manpages@ ]]; then cat doc/manual/src/command-ref/new-cli/SUMMARY.md; else echo "$$line"; fi; done > $@.tmp
 	@mv $@.tmp $@
 
-$(d)/src/command-ref/new-cli: $(d)/nix.json $(d)/generate-manpage.nix $(bindir)/nix
+$(d)/src/command-ref/new-cli: $(d)/nix.json $(d)/generate-manpage.nix $(nix_DIR)/nix
 	@rm -rf $@
 	$(trace-gen) $(nix-eval) --write-to $@ --expr 'import doc/manual/generate-manpage.nix (builtins.fromJSON (builtins.readFile $<))'
 
-$(d)/src/command-ref/conf-file.md: $(d)/conf-file.json $(d)/generate-options.nix $(d)/src/command-ref/conf-file-prefix.md $(bindir)/nix
+$(d)/src/command-ref/conf-file.md: $(d)/conf-file.json $(d)/generate-options.nix $(d)/src/command-ref/conf-file-prefix.md $(nix_DIR)/nix
 	@cat doc/manual/src/command-ref/conf-file-prefix.md > $@.tmp
 	$(trace-gen) $(nix-eval) --expr 'import doc/manual/generate-options.nix (builtins.fromJSON (builtins.readFile $<))' >> $@.tmp
 	@mv $@.tmp $@
 
-$(d)/nix.json: $(bindir)/nix
-	$(trace-gen) $(dummy-env) $(bindir)/nix __dump-args > $@.tmp
+$(d)/nix.json: $(nix_DIR)/nix
+	$(trace-gen) $(dummy-env) $(nix_DIR)/nix __dump-args > $@.tmp
 	@mv $@.tmp $@
 
-$(d)/conf-file.json: $(bindir)/nix
-	$(trace-gen) $(dummy-env) $(bindir)/nix show-config --json --experimental-features nix-command > $@.tmp
+$(d)/conf-file.json: $(nix_DIR)/nix
+	$(trace-gen) $(dummy-env) $(nix_DIR)/nix show-config --json --experimental-features nix-command > $@.tmp
 	@mv $@.tmp $@
 
-$(d)/src/expressions/builtins.md: $(d)/builtins.json $(d)/generate-builtins.nix $(d)/src/expressions/builtins-prefix.md $(bindir)/nix
+$(d)/src/expressions/builtins.md: $(d)/builtins.json $(d)/generate-builtins.nix $(d)/src/expressions/builtins-prefix.md $(nix_DIR)/nix
 	@cat doc/manual/src/expressions/builtins-prefix.md > $@.tmp
 	$(trace-gen) $(nix-eval) --expr 'import doc/manual/generate-builtins.nix (builtins.fromJSON (builtins.readFile $<))' >> $@.tmp
 	@cat doc/manual/src/expressions/builtins-suffix.md >> $@.tmp
 	@mv $@.tmp $@
 
-$(d)/builtins.json: $(bindir)/nix
-	$(trace-gen) $(dummy-env) NIX_PATH=nix/corepkgs=corepkgs $(bindir)/nix __dump-builtins > $@.tmp
+$(d)/builtins.json: $(nix_DIR)/nix
+	$(trace-gen) $(dummy-env) NIX_PATH=nix/corepkgs=corepkgs $(nix_DIR)/nix __dump-builtins > $@.tmp
 	@mv $@.tmp $@
 
 # Generate the HTML manual.