Bump actions/checkout from 2 to 4 #1624

Job	Run time
Bazel	5m 3s
Test Big Endian	6m 23s
macOS	6m 20s
ASAN	5m 57s
UBSAN	6m 23s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: true)	6m 8s
Ubuntu 18.04 (Build: Debug, Compilers: gcc-6/g++-6, Shared Library: true)	55s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: false)	6m 11s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	54s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: true)	6m 12s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	58s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: true)	51s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: false)	6m 12s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: false)	56s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	54s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	55s
	1h 1m 12s

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