internal/pkg: override "." for directory checksum

This makes the checksum consistent with the final resting state of artifact directories without incurring the cost of an extra pair of chown syscalls.

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

internal/pkg/pkg_test.go

@@ -404,7 +404,7 @@ func TestCache(t *testing.T) {
 				)
 			}
 			wantChecksum := pkg.MustDecode(
-				"1TL00Qb8dcqayX7wTO8WNaraHvY6b-KCsctLDTrb64QBCmxj_-byK1HdIUwMaFEP",
+				"qRN6in76LndiiOZJheHkwyW8UT1N5-f-bXvHfDvwrMw2fSkOoZdh8pWE1qhLk65b",
 			)
 			wantPathname := base.Append(
 				"identifier",
@@ -488,7 +488,7 @@ func TestCache(t *testing.T) {
 					},
 				}}}, nil, pkg.Checksum{}, &pkg.ChecksumMismatchError{
 					Got: pkg.MustDecode(
-						"GbjlYMcHQANdfwL6qNGopBF99IscPTvCy95HSH1_kIF3eKjFDSLP0_iUUT0z8hiw",
+						"CUx-3hSbTWPsbMfDhgalG4Ni_GmR9TnVX8F99tY_P5GtkYvczg9RrF5zO0jX9XYT",
 					),
 				}},
 
@@ -539,7 +539,7 @@ func TestCache(t *testing.T) {
 					fs.ModeSymlink | 0777,
 				)},
 			})
-		}, pkg.MustDecode("8OP6YxJAdRrhV2WSBt1BPD7oC_n2Qh7JqUMyVMoGvjDX83bDqq2hgVMNcdiBH_64")},
+		}, pkg.MustDecode("WVpvsVqVKg9Nsh744x57h51AuWUoUR2nnh8Md-EYBQpk6ziyTuUn6PLtF2e0Eu_d")},
 
 		{"pending", nil, func(t *testing.T, base *check.Absolute, c *pkg.Cache) {
 			c.SetStrict(true)