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/environment.scm

@@ -770,6 +770,13 @@ added to the container.
 
 Preserve environment variables whose name matches the one of the regexps in
 WHILE-LIST."
+  (define tmpfs
+    (file-system
+      (device "none")
+      (mount-point "/tmp")
+      (type "tmpfs")
+      (check? #f)))
+
   (define (optional-mapping->fs mapping)
     (and (file-exists? (file-system-mapping-source mapping))
          (file-system-mapping->bind-mount mapping)))
@@ -867,6 +874,7 @@ WHILE-LIST."
                       (writable? #f)))
                    reqs)))
             (file-systems (append %container-file-systems
+                                  (list tmpfs)
                                   (if network?
                                       (filter-map optional-mapping->fs
                                                   %network-file-mappings)

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.

tests/guix-environment-container.sh

@@ -195,6 +195,11 @@ guix environment --bootstrap --container --ad-hoc guile-bootstrap	\
      --writable-root							\
      -- guile -c '(mkdir "/whatever")'
 
+# /tmp itself is always writable.
+guix environment --bootstrap --container --ad-hoc guile-bootstrap \
+     -- guile -c '(mkdir "/tmp/foo")'
+
+
 # Check the exit code.
 
 abnormal_exit_code="