ci: Use reusable workflows for fmt and security_audit #741

Job	Run time
clippy	16s
msrv (ubuntu-latest, 1.63)	1m 39s
test (ubuntu-latest, nightly)	19s
msrv (windows-latest, 1.63)	2m 6s
test (ubuntu-latest, beta)	19s
test (ubuntu-latest, stable)	18s
test (windows-latest, nightly)	55s
test (windows-latest, beta)	37s
test (windows-latest, stable)	1m 48s
test (macos-latest, nightly)	21s
test (macos-latest, beta)	20s
test (macos-latest, stable)	13s
fmt / fmt	10s
security_audit / security_audit	2m 40s
	12m 1s

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