src: optimize trace_name allocation in Worker::Run #1260

Job	Run time
Build slim tarball	1m 33s
x86_64-linux: with shared libraries	29m 17s
x86_64-darwin: with shared libraries	38m 49s
aarch64-linux: with shared libraries	25m 34s
aarch64-darwin: with shared libraries	28m 46s
	2h 3m 59s

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