`om develop`: Use `FlakeUrl` without attrs for fetching `OmConfig` #1363

Job	Run time
main (x86_64-linux, false)	2m 19s
main (aarch64-darwin, false)	3m 51s
static-binary-check (ubuntu-latest)	13s
website	0s
static-binary-check (macos-latest)	6s
	6m 29s

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