Add back add_string_variable #380

Job	Run time
Address sanitizer	1m 44s
mpi-runner	3m 23s
rustfmt	39s
conda (ubuntu, conda-forge, stable)	1m 10s
conda (windows, conda-forge, stable)	3m 45s
static builds (ubuntu-latest, stable)	1m 32s
Spell checking (typos)	4s
conda (macos, conda-forge, stable)	1m 12s
test apt (stable)	3m 25s
static builds (windows-latest, stable-msvc)	6m 34s
static builds (windows-latest, stable-gnu)	6m 6s
test apt (beta)	4m 6s
static builds (macos-13, stable)	4m 12s
test apt (nightly)	45s
static builds (macos-latest, stable)	1m 26s
	40m 3s

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