legacy_strftime — UTC time, formated using the ISO C standard format
legacy_strftime function returns the current date and time using the given
format string. The format string follows strftime's datetime formatting.
This function has been provided to create backwards compatibility with Packer's
legacy JSON templates. However, we recommend that you upgrade your HCL Packer
template to use
together as soon as is convenient.
Note: If you are using a large number of builders, provisioners or post-processors, the strftime may be slightly different for each one because it is from when the plugin is launched not the initial Packer process. In order to avoid this and make the timestamp consistent across all plugins, set it as a user variable and then access the user variable within your plugins.