Bump actions/checkout from 2 to 4 #1618

Job	Run time
Test Big Endian	9m 19s
Bazel	5m 10s
macOS	14m 2s
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)	52s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: true)	50s
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)	53s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	51s
ASAN	6m 11s
UBSAN	6m 18s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: true)	19m 9s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: false)	19m 7s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: true)	9m 22s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: false)	9m 2s
	1h 43m 34s

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