Remove old build tag comments

Go is transitioning from the old '// +build' form to '//go:build', this
removes all uses of the old form, adding the new one where needed.

See https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md

internal/crypt/crypt_impl.go

@@ -1,5 +1,4 @@
 //go:build !darwin
-// +build !darwin
 
 // Copied from https://github.com/amoghe/go-crypt/blob/b3e291286513a0c993f7c4dd7060d327d2d56143/crypt_r.go
 // Original sources are under MIT license:

internal/crypt/crypt_impl_macos.go

@@ -1,5 +1,4 @@
 //go:build darwin
-// +build darwin
 
 package crypt
 

internal/crypt/crypt_test.go

@@ -1,5 +1,4 @@
 //go:build !darwin
-// +build !darwin
 
 package crypt