Preserve exit status of nix develop #44

Job	Run time
list-determinate-shells	14s
check (ubuntu-22.04)	21s
test-self (ubuntu-22.04)	26s
check (macos-13)	1m 24s
test-self (macos-13)	43s
test-determinate-shells (cpp)	23s
test-determinate-shells (default)	1m 49s
test-determinate-shells (example)	22s
test-determinate-shells (go)	21s
test-determinate-shells (haskell)	46s
test-determinate-shells (hook)	23s
test-determinate-shells (javascript)	21s
test-determinate-shells (multi)	24s
test-determinate-shells (python)	21s
test-determinate-shells (rust)	34s
test-determinate-shells (scala)	43s
	9m 35s

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