internal/pkg: create work directory

This is used for artifacts that cure into directories.

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

internal/pkg/pkg_test.go

@@ -263,7 +263,7 @@ func TestCache(t *testing.T) {
 			if _, _, err := c.LoadFile(zeroIdent); !reflect.DeepEqual(err, wantErrNonexistentZero) {
 				t.Fatalf("LoadFile: error = %#v, want %#v", err, wantErrNonexistentZero)
 			}
-		}, pkg.MustDecode("ZNSQH-mjhtIbFvi51lQ0UjatjoS8_5ILrBPNWlO2LWTq9P6MJEnekYzP0esUJnVr")},
+		}, pkg.MustDecode("G4u4W77C3u46oSAzwPTERKbS9h76iIvcd7Zl8p8Y6hTMb4_QGpH0Glg_DIJg-Usa")},
 	}
 	checkWithCache(t, testCases)
 }