rpmbuild - add build timestamp to rpm package name

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 40.87%. Comparing base (f616af2) to head (1e3b7c6). Report is 53 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #214 +/- ## ========================================== - Coverage 40.89% 40.87% -0.03% ========================================== Files 102 102 Lines 9659 9659 Branches 1425 1425 ========================================== - Hits 3950 3948 -2 - Misses 4867 4869 +2 Partials 842 842 ``` | [Flag](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/214/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET) | Coverage Δ | | |---|---|---| | [tests](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/214/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET) | `40.87% <ø> (-0.03%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET#carryforward-flags-in-the-pull-request-comment) to find out more.

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