[Bug fix] Change the default expression string for derived variables #5970

Job	Run time
git_checks	6s
format	29s
el8 (el8, icc, mpich)	24m 52s
windows (win2019, shared, msmpi)	22m 58s
el8 (el8, oneapi, mpich)	22m 44s
docker (ubuntu-bionic)	7m 16s
windows (win2022, shared, msmpi)	17m 29s
CodeQL (cpp)	21m 43s
windows (win2022, serial, windows-2022, vs2022)	12m 25s
windows (win2022, static, serial, windows-2022, vs2022)	13m 37s
macos (macos-13, xcode14_2, x64, shared, serial)	9m 46s
ubuntu (ubuntu20.04, gcc8, mpich)	6m 49s
macos (macos-14, xcode15_4, arm64, shared, serial)	7m 52s
ubuntu (ubuntu20.04, gcc9, mpich)	20m 23s
macos (macos-14, xcode15_4, arm64, static, serial)	4m 31s
ubuntu (ubuntu20.04, gcc10, mpich)	19m 34s
ubuntu (ubuntu20.04, clang6, mpich)	5m 58s
ubuntu (ubuntu20.04, clang10, mpich)	16m 48s
ubuntu (ubuntu20.04, gcc8, ompi)	36m 50s
ubuntu (ubuntu20.04, gcc8, serial)	11m 21s
ubuntu (ubuntu20.04, clang6, ompi)	26m 40s
ubuntu (ubuntu20.04, clang6, serial)	9m 37s
ubuntu (ubuntu20.04, gcc8, serial, static)	9m 26s
ubuntu (ubuntu22.04, rocm, serial, build_only)	4m 29s
ubuntu (ubuntu22.04, gcc11, mpich)	18m 52s
contract (examples)	2m 1s
contract (lammps)	4m 32s
contract (tau)	10m 43s
build_and_test	1s
	6h 9m 52s

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