Add lock semantics around dev_mode.IsEnabled to avoid data races (#42646)

Add lock semantics around dev_mode.IsEnabled to as a fix for
[this](https://github.com/fleetdm/fleet/actions/runs/23728512273) data
race

server/dev_mode/dev_mode.go

@@ -2,19 +2,32 @@ package dev_mode
 
 import (
 	"os"
+	"sync"
 	"testing"
 )
 
+// IsEnabled should be configured once at process startup (e.g., via flags) and then treated as read-only.
+// It must not be written from concurrent goroutines; SetOverride only affects enabledViaOverride/env overrides.
 var IsEnabled bool
 
+// enabledViaOverride is set by SetOverride and protected by mu so that it is
+// always observed consistently with envOverrides.
+var enabledViaOverride bool
+
+var mu sync.RWMutex
+
 var envOverrides = map[string]string{}
 
 type GetEnv func(name string) string
 
 func Env(name string) string {
-	if !IsEnabled {
+	mu.RLock()
+	defer mu.RUnlock()
+
+	if !IsEnabled && !enabledViaOverride {
 		return ""
 	}
+
 	if override, ok := envOverrides[name]; ok {
 		return override
 	}
@@ -30,14 +43,24 @@ func SetOverride(name string, value string, cleanup ...*testing.T) { // optional
 		})
 	}
 
-	IsEnabled = true // if we're setting overrides, we're in a test environment so want to turn dev mode on
+	mu.Lock()
+	defer mu.Unlock()
+
+	enabledViaOverride = true // if we're setting overrides, we're in a test environment so want to turn dev mode on
 	envOverrides[name] = value
 }
 
 func ClearOverride(name string) {
+	mu.Lock()
+	defer mu.Unlock()
+
 	delete(envOverrides, name)
 }
 
 func ClearAllOverrides() {
+	mu.Lock()
+	defer mu.Unlock()
+
+	enabledViaOverride = false
 	envOverrides = map[string]string{}
 }