tests: Actually run 'tests/guix-environment-container.sh'.

This test was skipped since the switch to Guile 3 because
'assert-container-features' would be inlined and thus accessing it with
@@ would fail with an unbound-variable error.

* guix/scripts/environment.scm (assert-container-features): Export.
* tests/guix-environment-container.sh: Use single '@'.

guix/scripts/environment.scm

@@ -48,7 +48,8 @@
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-37)
   #:use-module (srfi srfi-98)
-  #:export (guix-environment))
+  #:export (assert-container-features
+            guix-environment))
 
 ;; Protect some env vars from purification.  Borrowed from nix-shell.
 (define %precious-variables

tests/guix-environment-container.sh

@@ -24,7 +24,7 @@ set -e
 
 guix environment --version
 
-if ! guile -c '((@@ (guix scripts environment) assert-container-features))'
+if ! guile -c '((@ (guix scripts environment) assert-container-features))'
 then
     # User containers are not supported; skip this test.
     exit 77