Bump actions/checkout from 2 to 4 #1636

Job	Run time
Bazel	5m 9s
ASAN	5m 56s
Test Big Endian	9m 39s
macOS	19m 38s
UBSAN	6m 23s
Ubuntu 18.04 (Build: Debug, Compilers: gcc-6/g++-6, Shared Library: true)	23s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	24s
Ubuntu 18.04 (Build: Debug, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	21s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: true)	24s
Ubuntu 18.04 (Build: Release, Compilers: gcc-6/g++-6, Shared Library: false)	24s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: true)	22s
Ubuntu 18.04 (Build: Release, Compilers: clang-6.0/clang++-6.0, Shared Library: false)	24s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: true)	19m 4s
Windows 2019 (Generator: MinGW Makefiles, Shared Library: false)	19m 35s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: true)	9m 11s
Windows 2019 (Generator: Visual Studio 16 2019, Shared Library: false)	9m 42s
	1h 46m 59s

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