Use sha hash for checkout action and remove others #1404

Job	Run time
Tests / MSRV	2m 8s
Tests / MIPS (Big Endian)	2m 51s
Build / No deps	1m 16s
Build / Embedded	6s
Tests / Stable	2m 49s
Tests / WebAssembly	1m 57s
Tests / OS: macos-10.15 - stable-x86_64-apple-darwin	4m 47s
Tests / OS: ubuntu-20.04 - stable-x86_64-gnu	3m 9s
Tests / OS: windows-2019 - stable-x86_64-gnu	7m 12s
Tests / OS: windows-2019 - stable-x86_64-msvc	6m 5s
Tests / OS: macos-10.15 - beta-x86_64-apple-darwin	3m 8s
Tests / OS: ubuntu-20.04 - beta-x86_64-gnu	2m 31s
Tests / OS: windows-2019 - beta-x86_64-gnu	5m 23s
Tests / OS: windows-2019 - beta-x86_64-msvc	4m 7s
Tests / OS: macos-10.15 - nightly-x86_64-apple-darwin	3m 7s
Tests / OS: ubuntu-20.04 - nightly-x86_64-gnu	2m 27s
Tests / OS: windows-2019 - nightly-x86_64-gnu	4m 25s
Tests / OS: windows-2019 - nightly-x86_64-msvc	3m 2s
	1h 0m 30s

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