fix: replace remaining os.Setenv with t.Setenv in tests

Replace all os.Setenv calls with t.Setenv in test functions to ensure
proper test environment cleanup and better test isolation. This leaves
only legitimate application code and helper functions using os.Setenv.

internal/cli/cli_test.go

@@ -37,19 +37,9 @@ func TestCLIInstanceWithFs(t *testing.T) {
 func TestDetermineStateDir(t *testing.T) {
 	// Test the determineStateDir function from the secret package
 
-	// Save original environment and restore it after test
-	originalStateDir := os.Getenv(secret.EnvStateDir)
-	defer func() {
-		if originalStateDir == "" {
-			os.Unsetenv(secret.EnvStateDir)
-		} else {
-			os.Setenv(secret.EnvStateDir, originalStateDir)
-		}
-	}()
-
 	// Test with environment variable set
 	testEnvDir := "/test-env-dir"
-	os.Setenv(secret.EnvStateDir, testEnvDir)
+	t.Setenv(secret.EnvStateDir, testEnvDir)
 
 	stateDir := secret.DetermineStateDir("")
 	if stateDir != testEnvDir {