core: disable mounting disconnected private tmpfs on /var/tmp/ when DefaultDependencies=no

If DefaultDependencies=no, /var/ may not be mounted yet when the service
is being started. Previously, In such case, if the service has
PrivateTmp=disconnected, the service manager created /var/tmp/ on the
root filesystem and mounted the disconnected private tmpfs there. That
poluted the root filesystem and disturbed gpt-auto-generator on next
boot, as /var/ would not be empty anymore. See issue #37258.

This changes PrivateTmp=disconnected as the following:
- If DefaultDependencies=no and RootDirectory=/RootImage= are not set,
  then a private tmpfs is mounted _only_ on /tmp/, and set $TMPDIR=/tmp
  environment variable to suggest the service to use /tmp/.
- If DefaultDependencies=yes and RootDirectory=/RootImage= are not set,
  then implies RequiresMountsFor=/var/, though that is typically
  redundant, but anyway. Hence, we can safely mount /var/tmp/.
- Otherwise, i.e. when one of RootDirectory=/RootImage= is set, behaves
  as the same as the previous, as the private root filesystem for the
  service is explicitly prepared by the service manager, and we can
  safely mount a private tmpfs on /var/tmp/ without any extra
  dependencies.

Fixes #37258.

Co-authored-by: Mike Yuan <me@yhndnzj.com>

src/core/exec-invoke.c

@@ -1921,7 +1921,7 @@ static int build_environment(
         assert(cgroup_context);
         assert(ret);
 
-#define N_ENV_VARS 21
+#define N_ENV_VARS 22
         our_env = new0(char*, N_ENV_VARS + _EXEC_DIRECTORY_TYPE_MAX);
         if (!our_env)
                 return -ENOMEM;
@@ -2181,6 +2181,21 @@ static int build_environment(
                 our_env[n_env++] = x;
         }
 
+        assert(c->private_var_tmp >= 0 && c->private_var_tmp < _PRIVATE_TMP_MAX);
+        if (c->private_tmp != c->private_var_tmp) {
+                assert(c->private_tmp == PRIVATE_TMP_DISCONNECTED);
+                assert(c->private_var_tmp == PRIVATE_TMP_NO);
+
+                /* When private tmpfs is enabled only on /tmp/, then explicitly set $TMPDIR to suggest the
+                 * service to use /tmp/. */
+
+                x = strdup("TMPDIR=/tmp");
+                if (!x)
+                        return -ENOMEM;
+
+                our_env[n_env++] = x;
+        }
+
         assert(n_env < N_ENV_VARS + _EXEC_DIRECTORY_TYPE_MAX);
 #undef N_ENV_VARS
 
@@ -3589,6 +3604,7 @@ static int apply_mount_namespace(
                 .private_ipc = needs_sandboxing && exec_needs_ipc_namespace(context),
                 .private_pids = needs_sandboxing && exec_needs_pid_namespace(context) ? context->private_pids : PRIVATE_PIDS_NO,
                 .private_tmp = needs_sandboxing ? context->private_tmp : PRIVATE_TMP_NO,
+                .private_var_tmp = needs_sandboxing ? context->private_var_tmp : PRIVATE_TMP_NO,
 
                 .mount_apivfs = needs_sandboxing && exec_context_get_effective_mount_apivfs(context),
                 .bind_log_sockets = needs_sandboxing && exec_context_get_effective_bind_log_sockets(context),