Bump actions/checkout from 2 to 4 #1613

Job	Run time
ASAN	6m 15s
Bazel	5m 6s
Test Big Endian	9m 58s
macOS	14m 1s
UBSAN	6m 23s
Ubuntu 18.04 (Build: Debug, Compilers: gcc-6/g++-6, Shared Library: true)	50s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	51s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	50s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: true)	46s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: false)	47s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	49s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	49s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: true)	18m 31s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: false)	19m 5s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: true)	8m 47s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: false)	8m 52s
	1h 42m 40s

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