Fix golangci-lint failures in CI

internal/ptree/ptree_test.go

@@ -77,7 +77,7 @@ func TestWalkChildren(t *testing.T) {
 	require.Equal(t, "ready", string(ready[:]))
 
 	var numChildren int
-	ptree.WalkChildren(cmd.Process.Pid, func(pid int) {
+	ptree.WalkChildren(cmd.Process.Pid, func(_ int) {
 		numChildren++
 	})
 	assert.Equal(t, depth, numChildren)

pipe/command.go

@@ -201,7 +201,7 @@ func (s *commandStage) filterCmdError(err error) error {
 		// doesn't do anything on Windows, where the `Signaled()`
 		// method isn't implemented (it is hardcoded to return
 		// `false`).
-		ps, ok := eErr.ProcessState.Sys().(syscall.WaitStatus)
+		ps, ok := eErr.Sys().(syscall.WaitStatus)
 		if ok && ps.Signaled() &&
 			(ps.Signal() == syscall.SIGTERM || ps.Signal() == syscall.SIGKILL) {
 			return ctxErr

pipe/command_test.go

@@ -76,7 +76,6 @@ func TestCopyEnvWithOverride(t *testing.T) {
 	}
 
 	for _, ex := range examples {
-		ex := ex
 		t.Run(ex.label, func(t *testing.T) {
 			assert.ElementsMatch(t, ex.expectedResult,
 				copyEnvWithOverrides(ex.env, ex.overrides))

pipe/pipeline.go

@@ -28,6 +28,7 @@ type Env struct {
 // and is not reported to the caller.
 //
 //revive:disable:error-naming
+//nolint:staticcheck // ST1012: FinishEarly is the intentional name for this sentinel error
 var FinishEarly = errors.New("finish stage early")
 
 //revive:enable:error-naming
@@ -67,7 +68,7 @@ type Pipeline struct {
 	panicHandler StagePanicHandler
 }
 
-var emptyEventHandler = func(e *Event) {}
+var emptyEventHandler = func(_ *Event) {}
 
 type nopWriteCloser struct {
 	io.Writer

pipe/pipeline_test.go

@@ -473,7 +473,7 @@ func TestFunction(t *testing.T) {
 			pipe.Print("hello world"),
 			pipe.Function(
 				"farewell",
-				func(_ context.Context, _ pipe.Env, stdin io.Reader, stdout io.Writer) error {
+				func(_ context.Context, _ pipe.Env, _ io.Reader, _ io.Writer) error {
 					panic("this is a panic")
 				},
 			),
@@ -886,7 +886,6 @@ func TestErrors(t *testing.T) {
 			expectedErr: err1,
 		},
 	} {
-		tc := tc
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()