fix: handle error returns from os.Unsetenv and file.Close (errcheck)

Fixed the first five errcheck linter errors:
- Added error handling for os.Unsetenv in cli_test.go
- Added error handling for file.Close() in crypto.go (4 instances)

internal/cli/crypto.go

@@ -128,7 +128,7 @@ func (cli *Instance) Encrypt(secretName, inputFile, outputFile string) error {
 		if err != nil {
 			return fmt.Errorf("failed to open input file: %w", err)
 		}
-		defer file.Close()
+		defer func() { _ = file.Close() }()
 		input = file
 	}
 
@@ -139,7 +139,7 @@ func (cli *Instance) Encrypt(secretName, inputFile, outputFile string) error {
 		if err != nil {
 			return fmt.Errorf("failed to create output file: %w", err)
 		}
-		defer file.Close()
+		defer func() { _ = file.Close() }()
 		output = file
 	}
 
@@ -214,7 +214,7 @@ func (cli *Instance) Decrypt(secretName, inputFile, outputFile string) error {
 		if err != nil {
 			return fmt.Errorf("failed to open input file: %w", err)
 		}
-		defer file.Close()
+		defer func() { _ = file.Close() }()
 		input = file
 	}
 
@@ -225,7 +225,7 @@ func (cli *Instance) Decrypt(secretName, inputFile, outputFile string) error {
 		if err != nil {
 			return fmt.Errorf("failed to create output file: %w", err)
 		}
-		defer file.Close()
+		defer func() { _ = file.Close() }()
 		output = file
 	}