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/secret/secret_test.go

@@ -128,19 +128,9 @@ func TestPerSecretKeyFunctionality(t *testing.T) {
 	// Create an in-memory filesystem for testing
 	fs := afero.NewMemMapFs()
 
-	// Set up test environment variables
-	oldMnemonic := os.Getenv(EnvMnemonic)
-	defer func() {
-		if oldMnemonic == "" {
-			os.Unsetenv(EnvMnemonic)
-		} else {
-			os.Setenv(EnvMnemonic, oldMnemonic)
-		}
-	}()
-
 	// Set test mnemonic for direct encryption/decryption
 	testMnemonic := "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about"
-	os.Setenv(EnvMnemonic, testMnemonic)
+	t.Setenv(EnvMnemonic, testMnemonic)
 
 	// Set up a test vault structure
 	baseDir := "/test-config/berlin.sneak.pkg.secret"
@@ -312,9 +302,7 @@ func TestSecretGetValueWithEnvMnemonicUsesVaultDerivationIndex(t *testing.T) {
 
 	// Set up test mnemonic
 	testMnemonic := "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about"
-	originalEnv := os.Getenv(EnvMnemonic)
-	os.Setenv(EnvMnemonic, testMnemonic)
-	defer os.Setenv(EnvMnemonic, originalEnv)
+	t.Setenv(EnvMnemonic, testMnemonic)
 
 	// Create temporary directory for vaults
 	fs := afero.NewOsFs()