Preserve exit status of nix develop #47

Job	Run time
list-determinate-shells	14s
check (ubuntu-22.04)	21s
test-self (ubuntu-22.04)	22s
check (macos-13)	1m 29s
test-self (macos-13)	2m 36s
test-determinate-shells (cpp)	21s
test-determinate-shells (default)	1m 43s
test-determinate-shells (example)	24s
test-determinate-shells (go)	21s
test-determinate-shells (haskell)	47s
test-determinate-shells (hook)	20s
test-determinate-shells (javascript)	23s
test-determinate-shells (multi)	22s
test-determinate-shells (python)	23s
test-determinate-shells (rust)	35s
test-determinate-shells (scala)	38s
	11m 19s

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