disable payload signing for uploads by default #228

Job	Run time
docs	43s
fmt	7s
minrust	1m 14s
clippy	1m 58s
basic checks	1s
features	1m 34s
saniters	4m 20s
check deps with cargo-deny	17s
check-external-types (ubuntu-24.04)	1m 45s
Test S3 transfer manager HLL (ubuntu-24.04)	3m 12s
Test S3 transfer manager HLL (windows-2022)	6m 16s
Test S3 transfer manager HLL (macos-14)	3m 9s
Required checks pass	1s
	24m 37s

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