bump github action/checkout #371

Job	Run time
pre-ci	3s
v3.2.x-gh-ubuntu20-linux-gcc-lean	2m 12s
v3.2.x-gh-ubuntu20-linux-gcc	2m 23s
v3.2.x-gh-ubuntu20-linux-gcc-reproducible	2m 20s
v3.2.x-gh-ubuntu20-linux-gcc-O2-g3	3m 26s
v3.2.x-gh-ubuntu20-linux-clang-lean	4m 36s
v3.2.x-gh-ubuntu20-linux-clang	5m 31s
v3.2.x-gh-ubuntu20-linux-clang-altlibs	6m 11s
v3.2.x-gh-ubuntu20-linux-clang-reproducible	6m 39s
v3.2.x-gh-ubuntu20-linux-clang-sanitizer	7m 54s
v3.2.x-gh-ubuntu20-linux-clang-O2-g3	6m 30s
Merge preflight	1s
Merge into upstream	0s
	47m 46s

Provide feedback