rpk: only include tuner package files in linux

These files are only used in our tuner package, which only works for
linux.

Our other build targets (windows/darwin) should not care about these
files.

This also fixes an issue that prevented Mac users from running the
linter.

(cherry picked from commit 23333be)

Author: r-vasquez

src/go/rpk/pkg/cli/iotune/iotune.go

@@ -7,7 +7,7 @@
 // the Business Source License, use of this software will be governed
 // by the Apache License, Version 2.0
 
-//go:build !windows
+//go:build linux
 
 package iotune
 

src/go/rpk/pkg/system/grub.go

@@ -7,6 +7,8 @@
 // the Business Source License, use of this software will be governed
 // by the Apache License, Version 2.0
 
+//go:build linux
+
 package system
 
 import (

src/go/rpk/pkg/system/grub_test.go

@@ -7,6 +7,8 @@
 // the Business Source License, use of this software will be governed
 // by the Apache License, Version 2.0
 
+//go:build linux
+
 package system
 
 import (

src/go/rpk/pkg/tuners/aggregated_tunable.go

@@ -7,6 +7,8 @@
 // the Business Source License, use of this software will be governed
 // by the Apache License, Version 2.0
 
+//go:build linux
+
 package tuners
 
 func NewAggregatedTunable(tunables []Tunable) Tunable {

src/go/rpk/pkg/tuners/aggregated_tunable_test.go

@@ -7,6 +7,8 @@
 // the Business Source License, use of this software will be governed
 // by the Apache License, Version 2.0
 
+//go:build linux
+
 package tuners
 
 import (

src/go/rpk/pkg/tuners/aio.go

@@ -7,6 +7,8 @@
 // the Business Source License, use of this software will be governed
 // by the Apache License, Version 2.0
 
+//go:build linux
+
 package tuners
 
 import (

src/go/rpk/pkg/tuners/aio_test.go

@@ -7,6 +7,8 @@
 // the Business Source License, use of this software will be governed
 // by the Apache License, Version 2.0
 
+//go:build linux
+
 package tuners_test
 
 import (

src/go/rpk/pkg/tuners/check.go

@@ -7,6 +7,8 @@
 // the Business Source License, use of this software will be governed
 // by the Apache License, Version 2.0
 
+//go:build linux
+
 package tuners
 
 import (

src/go/rpk/pkg/tuners/checked_tunable.go

@@ -7,6 +7,8 @@
 // the Business Source License, use of this software will be governed
 // by the Apache License, Version 2.0
 
+//go:build linux
+
 package tuners
 
 import (

src/go/rpk/pkg/tuners/checked_tunable_test.go

@@ -7,6 +7,8 @@
 // the Business Source License, use of this software will be governed
 // by the Apache License, Version 2.0
 
+//go:build linux
+
 package tuners
 
 import (