all: adjust some build tags for plan9

I'm not saying it works, but it compiles.

Updates #5794

Change-Id: I2f3c99732e67fe57a05edb25b758d083417f083e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tsconnect/build-pkg.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !plan9
+
 package main
 
 import (

cmd/tsconnect/build.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !plan9
+
 package main
 
 import (

cmd/tsconnect/common.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !plan9
+
 package main
 
 import (

cmd/tsconnect/dev-pkg.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !plan9
+
 package main
 
 import (

cmd/tsconnect/dev.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !plan9
+
 package main
 
 import (

cmd/tsconnect/serve.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !plan9
+
 package main
 
 import (

cmd/tsconnect/tsconnect.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !plan9
+
 // The tsconnect command builds and serves the static site that is generated for
 // the Tailscale Connect JS/WASM client. Can be run in 3 modes:
 //   - dev: builds the site and serves it. JS and CSS changes can be picked up