Bump actions/checkout from 2 to 4 #1622

Job	Run time
Bazel	4m 57s
ASAN	5m 54s
Test Big Endian	9m 7s
macOS	9m 5s
UBSAN	5m 54s
Ubuntu 18.04 (Build: Debug, Compilers: gcc-6/g++-6, Shared Library: true)	36s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	58s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: true)	7m 41s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	55s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: false)	8m 56s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: true)	1s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: true)	7m 46s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: false)	1s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: false)	8m 13s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	14s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	13s
	1h 10m 31s

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