CI, sonic: Replace backend-arm64 docker build with native GHA runner, disable sonic for arm64 (#1794)

* CI: Replace docker amd64 with native runner, switch Docker test arch to amd64

* tests: Remove spaces and new lines from test JSON timestamp fields

* builds: Disable sonic for arm64 due to OKX unmarshal issue

* nits: Improve Dockerfile comment and fix build tag

encoding/json/common.go

@@ -1,6 +1,6 @@
 // json is an abstraction middleware package to allow switching between json encoder/decoder implementations
 // The default implementation is sonic.
-// Build with `sonic_off` or `386` tags to switch to golang.org/encoding/json.
+// Build with `sonic_off`, '386' or 'arm64' tags to switch to golang.org/encoding/json.
 package json
 
 import "encoding/json" //nolint:depguard // Acceptable use in gct json wrapper

encoding/json/json.go

@@ -1,4 +1,4 @@
-//go:build sonic_off || 386
+//go:build sonic_off || 386 || arm64
 
 package json
 

encoding/json/sonic.go

@@ -1,4 +1,4 @@
-//go:build !sonic_off && !386
+//go:build !sonic_off && !386 && !arm64
 
 package json