PMM-13400 Improve cgroup write check and encryption setup

Suppress lint warnings in cgroup write test and simplify error handling. Add `isTest` function to detect test environment and update encryption key path logic accordingly.

agent/utils/cgroups/CgroupWritableCheck.go

@@ -26,13 +26,14 @@ func IsCgroupsWritable() bool {
 	testFile := filepath.Join(cgroupDir, "test_write")
 
 	// Attempt to create and remove a test file to check for write access
-	file, err := os.Create(testFile)
+	file, err := os.Create(testFile) //nolint:gosec
 	if err != nil {
 		return false // Not writable
 	}
 	defer func() {
-		file.Close()
-		os.Remove(testFile) // Clean up the test file
+		// Clean up the test file
+		file.Close()        //nolint:errcheck
+		os.Remove(testFile) //nolint:errcheck
 	}()
 
 	return true // Writable

managed/utils/encryption/encryption.go

@@ -21,7 +21,6 @@ import (
 	"encoding/base64"
 	"fmt"
 	"os"
-	"runtime"
 	"slices"
 	"strings"
 	"sync"
@@ -72,14 +71,24 @@ type QueryValues struct {
 	WhereValues [][]any
 }
 
+func isTest() bool {
+	// Check if tests are running by inspecting os.Args
+	for _, arg := range os.Args {
+		if strings.HasPrefix(arg, "-test.") {
+			return true
+		}
+	}
+	return false
+}
+
 // New creates an encryption; if key on path doesn't exist, it will be generated.
 func New() *Encryption {
 	e := &Encryption{}
 	customKeyPath := os.Getenv("PMM_ENCRYPTION_KEY_PATH")
 	if customKeyPath != "" {
 		e.Path = customKeyPath
 	} else {
-		if runtime.GOOS == "darwin" { // for development on macOS
+		if isTest() {
 			e.Path = "./encryption.key"
 		} else {
 			e.Path = DefaultEncryptionKeyPath