spindle/engines/nixery: make $HOME a separate directory

Otherwise, when programs create files in $HOME, others that check the Git
repo's state may be sad (or at least emit warnings in response to) it
being dirty.

Signed-off-by: Winter <[e-mail]>

spindle/engines/nixery/engine.go

···
 
 const (
 	workspaceDir = "/tangled/workspace"
+	homeDir      = "/tangled/home"
 )
 
 type cleanupFunc func(context.Context) error
···
 	}
 
 	mkExecResp, err := e.docker.ContainerExecCreate(ctx, resp.ID, container.ExecOptions{
-		Cmd:          []string{"mkdir", "-p", workspaceDir},
+		Cmd:          []string{"mkdir", "-p", workspaceDir, homeDir},
 		AttachStdout: true, // NOTE(winter): pretty sure this will make it so that when stdout read is done below, mkdir is done. maybe??
 		AttachStderr: true, // for good measure, backed up by docker/cli ("If -d is not set, attach to everything by default")
 	})
···
 	for k, v := range step.environment {
 		envs.AddEnv(k, v)
 	}
-	envs.AddEnv("HOME", workspaceDir)
+	envs.AddEnv("HOME", homeDir)
 	e.l.Debug("envs for step", "step", step.Name, "envs", envs.Slice())
 
 	mkExecResp, err := e.docker.ContainerExecCreate(ctx, addl.container, container.ExecOptions{