Bump actions/checkout from 2 to 4 #1620

Job	Run time
Bazel	5m 4s
ASAN	6m 10s
Test Big Endian	9m 20s
macOS	10m 0s
UBSAN	6m 8s
Ubuntu 18.04 (Build: Debug, Compilers: gcc-6/g++-6, Shared Library: true)	45s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: true)	9m 48s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	48s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: false)	9m 46s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	48s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: true)	9m 18s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: true)	48s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: false)	49s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: false)	8m 54s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	47s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	47s
	1h 20m 0s

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