Preserve exit status of nix develop #42

Job	Run time
list-determinate-shells	15s
test-self (ubuntu-22.04)	22s
check (ubuntu-22.04)	21s
test-self (macos-13)	1m 26s
check (macos-13)	37s
test-determinate-shells (cpp)	22s
test-determinate-shells (default)	1m 49s
test-determinate-shells (example)	22s
test-determinate-shells (go)	24s
test-determinate-shells (haskell)	45s
test-determinate-shells (hook)	20s
test-determinate-shells (javascript)	20s
test-determinate-shells (multi)	22s
test-determinate-shells (python)	21s
test-determinate-shells (rust)	35s
test-determinate-shells (scala)	36s
	9m 17s

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