pkg/eval: Fix EPIPE on Windows.

The syscall package exposes an EPIPE on Windows, but it's not what Windows APIs
return. The error number 232 is what is semantically EPIPE.

pkg/eval/port.go

@@ -6,7 +6,6 @@ import (
 	"io"
 	"os"
 	"sync"
-	"syscall"
 
 	"src.elv.sh/pkg/eval/errs"
 	"src.elv.sh/pkg/eval/vals"
@@ -292,7 +291,7 @@ func (bo byteOutput) WriteString(s string) (int, error) {
 
 func convertReaderGone(err error) error {
 	if pathErr, ok := err.(*os.PathError); ok {
-		if pathErr.Err == syscall.EPIPE {
+		if pathErr.Err == epipe {
 			return errs.ReaderGone{}
 		}
 	}

pkg/eval/port_unix.go

@@ -0,0 +1,7 @@
+//+build !windows,!plan9,!js
+
+package eval
+
+import "syscall"
+
+var epipe = syscall.EPIPE

pkg/eval/port_windows.go

@@ -0,0 +1,10 @@
+package eval
+
+import "syscall"
+
+// Error number 232 is what Windows returns when trying to write on a pipe who
+// reader has gone. The syscall package defines an EPIPE on Windows, but that's
+// not what Windows API actually returns.
+//
+// https://docs.microsoft.com/en-us/windows/win32/debug/system-error-codes--0-499-
+var epipe = syscall.Errno(232)