fix: Force PLATFORMS environment variable when we build Elastic Agent…

… dependencies on arm64 (#26415)

* fix: build elastic agent dependencies on the architecture you are running

* fix: format

* fix: build ARM on ARM

* fix: format

* chore: backup the platforms env var

(cherry picked from commit 5488dcf)

x-pack/elastic-agent/magefile.go

@@ -566,6 +566,16 @@ func packageAgent(requiredPackages []string, packagingFn func()) {
 		}
 
 		os.Setenv(agentDropPath, dropPath)
+		if runtime.GOARCH == "arm64" {
+			const platformsVar = "PLATFORMS"
+			oldPlatforms := os.Getenv(platformsVar)
+			os.Setenv(platformsVar, runtime.GOOS+"/"+runtime.GOARCH)
+			if oldPlatforms != "" {
+				defer os.Setenv(platformsVar, oldPlatforms)
+			} else {
+				defer os.Unsetenv(oldPlatforms)
+			}
+		}
 
 		// cleanup after build
 		defer os.RemoveAll(dropPath)