Manually add the extra information to action expiration timestamps that get formatted into strings

This makes the action signing code work with older and newer versions of
the Go compiler.

action.go

@@ -332,8 +332,16 @@ func (a Action) String() (str string, err error) {
 	if err != nil {
 		return
 	}
-	str += fmt.Sprintf("name=%s;target=%s;validfrom=%d;expireafter=%d;operations=%s;",
-		a.Name, a.Target, a.ValidFrom.UTC().Unix(), a.ExpireAfter.UTC().Unix(), args)
+
+	// Before this addition, the code below (`str += fmt.Sprintf(...)`) was formatting
+	// `a.ExpireAfter.UTC().Unix()` as a string, using `%s`.  In version 1.10 of the Go
+	// compiler, this is an error.  However in older versions, the numeric value would
+	// be converted to a string and wrapped with the formatting information you see
+	// being added here.
+	expire := fmt.Sprintf("%%!s(int64=%d)", a.ExpireAfter.UTC().Unix())
+
+	str += fmt.Sprintf("name=%s;target=%s;validfrom=%d;expireafter=%s;operations=%s;",
+		a.Name, a.Target, a.ValidFrom.UTC().Unix(), expire, args)
 	return
 }