internal/pkg: pick up $TERM if attaching stdin

This improves behaviour of some programs.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/exec.go

@@ -579,6 +579,11 @@ func (c *Cache) EnterExec(
 	z.Stdin, z.Stdout, z.Stderr = stdin, stdout, stderr
 	z.Path, z.Args = path, args
 	z.RetainSession = retainSession
+	if stdin == os.Stdin {
+		if s, ok := os.LookupEnv("TERM"); ok {
+			z.Env = append(z.Env, "TERM="+s)
+		}
+	}
 
 	if err = z.Start(); err != nil {
 		return