integration tests: pass XDG_CACHE_HOME to agent tests

The integration test setup was not passing through the XDG_CACHE_HOME
environment variable to the test environment, causing tests to fall
back to $HOME/.cache for caching downloaded files. This caused test
failures in environments where $HOME/.cache is not writable but
XDG_CACHE_HOME points to a writable location.

This change ensures that if XDG_CACHE_HOME is set in the test runner's
environment, it is passed through to the integration test environment,
allowing tests to respect the XDG Base Directory specification.

Assisted-by: Claude Code

Author: zaneb

cmd/openshift-install/internal_integration_test.go

@@ -112,6 +112,9 @@ func runIntegrationTest(t *testing.T, testFolder string) {
 				}
 			}
 			e.Vars = append(e.Vars, fmt.Sprintf("RELEASE_IMAGE=%s", pullspec))
+			if xdgCacheHome, ok := os.LookupEnv("XDG_CACHE_HOME"); ok && xdgCacheHome != "" {
+				e.Vars = append(e.Vars, fmt.Sprintf("XDG_CACHE_HOME=%s", xdgCacheHome))
+			}
 			// When AUTH_FILE is set in the CI integration-tests job
 			if authFilePath, ok := os.LookupEnv("AUTH_FILE"); ok && authFilePath != "" {
 				workDir := e.Getenv("WORK")