Bump actions/checkout from 2 to 4 #1625

Job	Run time
macOS	16m 19s
Test Big Endian	9m 37s
ASAN	6m 22s
Bazel	5m 24s
UBSAN	6m 27s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: true)	18m 47s
Ubuntu 18.04 (Build: Debug, Compilers: gcc-6/g++-6, Shared Library: true)	11m 45s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: false)	19m 7s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	9m 3s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: true)	9m 31s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	9m 18s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: false)	9m 51s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: true)	15m 20s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: false)	15m 8s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	14m 1s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	13m 57s
	3h 9m 57s

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