clear Rx Time
The time this message was received by the esp32 (secs since 1970).
Note: this field is _never_ sent on the radio link itself (to save space) Times
are typically not sent over the mesh, but they will be added to any Packet
(chain of SubPacket) sent to the phone (so the phone can know exact time of reception)
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fixed32 rx_time = 7;
Return
This builder for chaining.