Accept vendor-specified build date if .git/ is unavailable

Fixes #10053

common/hugo/hugo.go

@@ -38,6 +38,8 @@ const (
 )
 
 var (
+	// buildDate allows vendor-specified build date when .git/ is unavailable.
+	buildDate string
 	// vendorInfo contains vendor notes about the current build.
 	vendorInfo string
 )

common/hugo/version.go

@@ -153,6 +153,10 @@ func BuildVersionString() string {
 	osArch := bi.GoOS + "/" + bi.GoArch
 
 	date := bi.RevisionTime
+	if date == "" {
+		// Accept vendor-specified build date if .git/ is unavailable.
+		date = buildDate
+	}
 	if date == "" {
 		date = "unknown"
 	}