Avoid copy in `SentryEnvelopeItem.envelopeItemStream` #1347

Job	Run time
cancel-previous-workflow	2s
android \| ubuntu-latest \| stable	1m 1s
android \| ubuntu-latest \| beta	1m 2s
linux \| ubuntu-latest \| stable	1m 0s
linux \| ubuntu-latest \| beta	59s
ios \| macos-latest \| stable	1s
macos \| macos-latest \| stable	1s
windows \| windows-latest \| stable	1m 9s
analyze / cancel-previous-workflow	2s
analyze / analyze	54s
analyze / package-analysis	1m 4s
	7m 15s

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