Configure Open MPI shmem defaults on macOS #4941

Job	Run time
ci-scope	3s
pre-commit / pre-commit	2m 19s
pre-commit / link-check	36s
windows / clang-build	1m 47s
windows / msvc-build (Debug)	1m 55s
mac / clang-build (Release)	1m 54s
windows / msvc-build (Release)	1m 55s
ubuntu / gcc-build (ubuntu-24.04, Release)	1m 43s
ubuntu / clang-build (ubuntu-24.04)	1m 42s
ubuntu / gcc-build (ubuntu-24.04, Debug)	1m 42s
mac / clang-build (Debug)	1m 45s
ubuntu / clang-test	1s
ubuntu / clang-sanitizer-build	1s
ubuntu / clang-test-extended	1s
ubuntu / clang-sanitizer-test	1s
ubuntu / clang-sanitizer-test-extended	1s
ubuntu / gcc-test	1s
ubuntu / gcc-test-extended	1s
ubuntu / gcc-build-codecov	1s
windows / clang-test	1s
windows / clang-test-extended	1s
mac / clang-test	1s
mac / clang-test-extended	1s
windows / msvc-test	1s
windows / msvc-test-extended	1s
perf	0s
pages	-1s
	17m 34s

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