Load the manifest check downloads from memory, not a temp file (closes #223)

check given a URL wrote the manifest it downloaded to a file in the temp
directory before loading it, so it failed wherever that directory is
missing or cannot be written, such as the scratch image script/cibuild
builds. It now keeps the manifest in memory, as fetch, list and export
do. NewChecker reads a manifest handed to it as a reader through the new
CheckerOptions.Manifest field, in place of ManifestPath, because it
otherwise reads the manifest from the filesystem holding the files it
checks.

Model: opus-5-5

internal/cli/entry_test.go

@@ -1167,6 +1167,29 @@ func TestCheckURLManifestResolvesEntriesAgainstCurrentDirectory(t *testing.T) {
 	assert.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
 }
 
+// TestCheckURLManifestWithoutTempDirectory checks the tree under --base
+// against a manifest given by URL while TMPDIR names a directory that does
+// not exist. check keeps the manifest it downloads in memory, so it passes.
+// Setting TMPDIR keeps it from running in parallel with other tests.
+func TestCheckURLManifestWithoutTempDirectory(t *testing.T) {
+	files := map[string][]byte{testFileTxt: []byte("hello")}
+
+	server := httptest.NewServer(fetchTestHandler(manifestOf(t, files), files))
+	defer server.Close()
+
+	base := t.TempDir()
+	require.NoError(t,
+		os.WriteFile(filepath.Join(base, testFileTxt), files[testFileTxt], 0o600))
+
+	t.Setenv("TMPDIR", filepath.Join(t.TempDir(), "missing"))
+
+	opts := testOpts([]string{
+		testApp, cmdCheck, testFlagNoExtra, testFlagBase, base,
+		server.URL + "/" + defaultManifestName,
+	}, afero.NewOsFs())
+	assert.Equal(t, 0, runCLI(opts), "stderr: %s", testStderr(t, opts))
+}
+
 // unlistableDirFs is a filesystem on which one directory cannot be listed.
 type unlistableDirFs struct {
 	afero.Fs