pageserver: respect no_sync in `VirtualFile` #23162

Job	Run time
check-permissions / check-permissions	1s
check-macos-build	0s
check-build-tools-image / check-image	5s
build-build-tools-image / check-image / check-image	10s
build-build-tools-image / build-image	0s
build-build-tools-image / merge-images	0s
gather-rust-build-stats	0s
	16s

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