cmd/go: skip TestScript/build_plugin_non_main on platforms that do not support -buildmode=plugin

Fixes #42474

Change-Id: I1550b44b92cd272854e2f17493245a14e3d39f41
GitHub-Last-Rev: 948d017
GitHub-Pull-Request: #42475
Reviewed-on: https://go-review.googlesource.com/c/go/+/268737
Reviewed-by: Bryan C. Mills <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>
Trust: Jay Conrod <[email protected]>

Author: XiaodongLoong

src/cmd/go/testdata/script/build_plugin_non_main.txt

@@ -1,7 +1,5 @@
-# Plugins are only supported on linux,cgo (!riscv64) and darwin,cgo.
-[!linux] [!darwin] skip
-[linux] [riscv64] skip
-[!cgo] skip
+# Plugins are not supported on all platforms.
+[!buildmode:plugin] skip
 
 go build -n testdep
 ! go build -buildmode=plugin testdep