environment, home: Make /tmp writable.

Fixes <https://issues.guix.gnu.org/78272>.

Fixes a bug caused by commit ce363c1dc7,
which inadvertently made /tmp read-only as well.

* guix/scripts/environment.scm (launch-environment/container)[tmpfs]:
New variable.
Add it to ‘file-systems’.
* guix/scripts/home.scm (spawn-home-container): Add /tmp file system
to #:mounts.
* tests/guix-environment-container.sh: Add test.

Reported-by: Tomas Volf <~@wolfsden.cz>
Change-Id: I5cb449e6d0c213d4b5866cbcbd14990437a7d4c5

guix/scripts/home.scm

@@ -353,7 +353,12 @@ immediately.  Return the exit status of the process in the container."
    #:namespaces (if network?
                     (delq 'net %namespaces)       ; share host network
                     %namespaces)
-   #:mounts (list (file-system
+   #:mounts (list (file-system                    ;writable /tmp
+                    (device "none")
+                    (mount-point "/tmp")
+                    (type "tmpfs")
+                    (check? #f))
+                  (file-system
                     (device "none")
                     (mount-point
                      (in-vicinity "/run/user"     ;for shepherd & co.