Bump actions/checkout from 2 to 4 #1626

Job	Run time
Test Big Endian	9m 30s
macOS	14m 18s
Bazel	5m 23s
ASAN	5m 56s
UBSAN	6m 21s
Ubuntu 18.04 (Build: Debug, Compilers: gcc-6/g++-6, Shared Library: true)	51s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	51s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: true)	18m 53s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	55s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: false)	18m 53s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: true)	54s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: true)	9m 31s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: false)	49s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: false)	9m 28s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	52s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	52s
	1h 44m 17s

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