Replace `date --rfc-3339=seconds` with macOS/BSD compatible equivalent

[elliotwutingfeng]

On macOS, running make ci would give error message date: illegal option -- -.

This is because date --rfc-3339=seconds is exclusive to GNU date.

On macOS/BSD systems, this is not supported by their variant of date.

A suitable equivalent that works on both macOS/BSD and Linux is date "+%F %T%:z".

References

• https://wiki.gentoo.org/wiki/Systemwide_RFC-3339_Dateformat
• https://forums.freebsd.org/threads/this-script-does-not-work-in-freebsd-beacuse-of-date.20458