home: services: shells: Double-quote environment variable values.

Fixes <https://issues.guix.gnu.org/56540>.

Until now, environment variable values were emitted unquoted, producing
invalid shell code if the value contains spaces for example.

* gnu/home/services/shells.scm (serialize-posix-env-vars): Define
'shell-quote' procedure in staged code and use it for #$value.
* tests/guix-home.sh: Add test for PS1 variable with a value containing
spaces.

tests/guix-home.sh

@@ -82,6 +82,8 @@ trap 'chmod -Rf +w "$test_directory"; rm -rf "$test_directory"' EXIT
    (simple-service 'home-bash-service-extension-test
                    home-bash-service-type
                    (home-bash-extension
+                    (environment-variables
+                      '(("PS1" . "$GUIX_ENVIRONMENT λ ")))
                     (bashrc
                      (list
                       (plain-file
@@ -138,6 +140,7 @@ EOF
 # dot-bashrc test file for guix home
 # the content of bashrc-test-config.sh"
     grep -q "the content of ~/.config/test.conf" "${HOME}/.config/test.conf"
+    grep '^export PS1="\$GUIX_ENVIRONMENT λ "$' "${HOME}/.bash_profile"
 
     # This one should still be here.
     grep "stay around" "$HOME/.config/random-file"