attestation: only enable for developers outside CI #28620

Job	Run time
syntax	48s
formula audit	6m 29s
tap syntax	4m 37s
docker	4m 38s
cask audit	3m 1s
test default formula (Ubuntu 22.04)	4m 27s
tests (online)	9m 4s
test default formula (Ubuntu 20.04)	6m 11s
update-test (Ubuntu 22.04)	24s
tests (generic OS)	6m 3s
vendored gems	46s
test default formula (macOS 13 x86_64)	2m 38s
update-test (macOS 13)	28s
tests (Ubuntu 22.04)	6m 6s
test default formula (macOS 14 arm64)	1m 12s
tests (Ubuntu 20.04)	7m 27s
tests (macOS 13 x86_64)	7m 3s
tests (macOS 14 arm64)	4m 12s
	1h 15m 34s

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