ci: Use reusable workflows for fmt and security_audit #866

Job	Run time
msrv	44s
test (stable)	1m 22s
test (beta)	1m 21s
clippy	22s
test (nightly)	1m 16s
loom	41s
test (nightly, i686-unknown-linux-gnu)	1m 28s
miri	3m 41s
fmt / fmt	11s
security_audit / security_audit	2m 42s
	13m 48s

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