fix(dist): don't accidentally include old files in release

When releasing quick-lint-js version 2.17.0, I accidentally included
some files from the 2.16.0 release. This is because I had stale builds
and signed-builds folders from when I released 2.16.0.

Update the release script to complain if an old builds or
signed-builds folder is found.

dist/release.go

@@ -165,6 +165,13 @@ var Steps []Step = []Step{
 		},
 	},
 
+	Step{
+		Title: "Check builds folder",
+		Run: func() {
+			EnsureEmptyDirectory("builds")
+		},
+	},
+
 	Step{
 		Title: "Download builds",
 		Run: func() {
@@ -177,6 +184,13 @@ var Steps []Step = []Step{
 		},
 	},
 
+	Step{
+		Title: "Check signed-builds folder",
+		Run: func() {
+			EnsureEmptyDirectory("signed-builds")
+		},
+	},
+
 	Step{
 		Title: "Sign the build artifacts",
 		Run: func() {
@@ -684,6 +698,28 @@ func UpdateDebianChangelog(changelogFilePath string, versionInfo VersionFileInfo
 	return nil
 }
 
+func EnsureEmptyDirectory(path string) {
+Retry:
+	err := os.Mkdir(path, 0700)
+	if err == nil {
+		// A newly-created directory is empty.
+		return
+	}
+	if !os.IsExist(err) {
+		// Unknown error. Report it to the user.
+		Stopf("%v", err)
+	}
+	entries, readError := os.ReadDir(path)
+	directoryIsEmpty := readError == nil && len(entries) == 0
+	if directoryIsEmpty {
+		return
+	}
+
+	fmt.Printf("Error: A '%s' folder already exists. Delete it then type 'done'.\n", path)
+	WaitForDone()
+	goto Retry
+}
+
 func StringLines(s string) []string {
 	return strings.Split(s, "\n")
 }