add option to use UTC-formatted time when using BUILD_DATE_FORMATTED

[Blackbaud-MikeLueders]

Eliminates potential non-increasing build numbers when using BUILD_DATE_FORMATTED caused by daylight savings time

[jenkinsadmin]

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[DenizThatMenace]

Your pull-request looks good except for some minor typos. (I added comments above.)

However, I fear the result might be a little surprising in some cases.

In particularly, if BUILD_DATE_FORMATTED is used along with e.g. BUILD_DAY or BUILDS_TODAY the output might not be what one expects if your "UTC" option is set.
The formatted build-date shows the UTC-time, while the current "build day" and the "number of builds today" use the local time and therefore display wrong values regarding the UTC-time.

Therefore, I would recommend, you change your pull-request (or provide a new one), so that setting the UTC-option has correct impact on all variables, not only on BUILD_DATE_FORMATTED.