Use unix build constraint.

Now that we floor on Go 1.19, take advantage of the unix build constraint:
https://tip.golang.org/doc/go1.19#go-unix

Signed-off-by: Nuno Cruces <ncruces@users.noreply.github.com>

imports/wasi_snapshot_preview1/wasi_stdlib_unix_test.go

@@ -1,4 +1,4 @@
-//go:build unix || linux || darwin
+//go:build unix
 
 package wasi_snapshot_preview1_test
 

internal/sysfs/file_unix.go

@@ -1,4 +1,4 @@
-//go:build unix || darwin || linux
+//go:build unix
 
 package sysfs
 

internal/sysfs/file_unsupported.go

@@ -1,4 +1,4 @@
-//go:build !unix && !linux && !darwin && !windows
+//go:build !unix && !windows
 
 package sysfs