Add back add_string_variable #381

Job	Run time
Address sanitizer	1m 23s
conda (ubuntu, conda-forge, stable)	1m 29s
rustfmt	1m 12s
mpi-runner	1m 41s
conda (windows, conda-forge, stable)	3m 5s
conda (macos, conda-forge, stable)	1m 1s
static builds (ubuntu-latest, stable)	1m 28s
test apt (stable)	46s
static builds (windows-latest, stable-msvc)	5m 34s
Spell checking (typos)	4s
test apt (beta)	41s
static builds (windows-latest, stable-gnu)	6m 2s
test apt (nightly)	42s
static builds (macos-13, stable)	3m 40s
static builds (macos-latest, stable)	1m 47s
	30m 35s

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