website: Makefile.am: use common guix shell command.

This change has several goals:

- It reduces code duplication. This also makes it easier to check that
  all the commands using Guix use the same revision and system, which
  are supposed to be common to the use of Guix. Unifying the Guix
  revision between the website and the rest of GNU Boot will be done
  later on.

- It reduce the size of the commands, which also help reduces the
  indentation and/or increase readability.

Guix users typically run "guix shell [arguments] -- [command]", and
here we abstract away some GNU Boot specific parts like using Guix
1.4.0 and i686-linux, so it makes sense to abstract them.

The --container argument is also specific to GNU Boot as it avoids
potentially leaks between the host and the container (which we want to
avoid for increased reproducibility across different host
distributions), however people used to guix shell will typically
expect to select between --container or not.

In order to more easily enforce --container and make it clear that we
use it, we named the variable GUIX_SHELL_CONTAINER.

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
Acked-by: Adrien Bourmault <neox@gnu.org>

website/Makefile.am

@@ -27,15 +27,17 @@ endif
 index.html: index.html.tmpl
 	sed -e "s#WEBSITE_PREFIX#$(WEBSITE_PREFIX)#g" "$^" > "$@"
 
-if WANT_GUIX
-DOT_CMD = \
-guix time-machine \
+GUIX_SHELL_CONTAINER = guix time-machine \
 	--commit=$(GUIX_REVISION) \
 	-- \
 	shell \
-		--system=i686-linux \
-		--container \
-		graphviz \
+	--system=i686-linux \
+	--container \
+	$(SENTINEL)
+
+if WANT_GUIX
+DOT_CMD = $(GUIX_SHELL_CONTAINER) \
+	graphviz \
 	-- \
 	dot
 else
@@ -74,24 +76,20 @@ site.cfg: site.cfg.tmpl
 	sed -e "s#WEBSITE_PREFIX#$(WEBSITE_PREFIX)#g" "$^" > "$@"
 
 if WANT_GUIX
-BUILD_SH_CMD = \
-guix time-machine \
-	--commit=c15a1cd88f80a90437f4b0159f22dfc84b9e6851 \
-	-- shell \
-		--container \
-		--network \
-		--emulate-fhs \
-		$(GUIX_SHARE_OPTIONS) \
-		bash \
-		coreutils \
-		findutils \
-		git \
-		grep \
-		nss-certs \
-		pandoc \
-		sed \
-		-- \
-		./build.sh
+BUILD_SH_CMD = $(GUIX_SHELL_CONTAINER) \
+	--network \
+	--emulate-fhs \
+	$(GUIX_SHARE_OPTIONS) \
+	bash \
+	coreutils \
+	findutils \
+	git \
+	grep \
+	nss-certs \
+	pandoc \
+	sed \
+	-- \
+	./build.sh
 else
 BUILD_SH_CMD = ./build.sh
 endif
@@ -100,9 +98,7 @@ build: site.cfg pages/footer.include
 	$(BUILD_SH_CMD) $(BUILD_OPTIONS)
 
 if WANT_GUIX
-CHECK_SH_CMD = \
-guix shell \
-	--container \
+CHECK_SH_CMD = $(GUIX_SHELL_CONTAINER) \
 	--network \
 	--emulate-fhs \
 	bash \
@@ -138,9 +134,7 @@ help:
 		"website.tar.gz # Create a tarball of the website"
 
 if WANT_GUIX
-SERVE_SH_CMD = \
-guix shell \
-	--container \
+SERVE_SH_CMD = $(GUIX_SHELL_CONTAINER) \
 	--network \
 	--emulate-fhs \
 	bash \