Add dependabot for github actions #616

Job	Run time
clippy	59s
test (msrv, ubuntu-20.04, 1.77.0)	1m 0s
test (stable, ubuntu-20.04, stable)	1m 3s
documentation	1m 22s
benchmarks	1m 47s
test (beta, ubuntu-20.04, beta)	1m 2s
Miri	2m 26s
test (nightly, ubuntu-20.04, nightly)	1m 17s
Optional features (approx selected)	31s
test (macos, macos-latest, stable)	1m 23s
Optional features (none selected)	29s
rustfmt	16s
test (win-msvc, windows-2019, stable)	1m 48s
static SuiteSparse (stable, ubuntu-20.04, stable)	41s
test (win-gnu, windows-2019, stable)	1m 48s
static SuiteSparse (macos, macos-latest, stable)	41s
static SuiteSparse (win-msvc, windows-2019, stable)	1m 18s
static SuiteSparse (win-gnu, windows-2019, stable)	1m 24s
	21m 15s

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