APPS/pkey: fix case where infile and outfile are the same #2199

Job	Run time
basic_gcc	5m 35s
check_docs	54s
check_update	2m 0s
check-ansi	2m 34s
enable_non-default_options	4m 31s
basic_clang	5m 58s
address_ub_sanitizer	10m 48s
full_featured	5m 57s
fuzz_tests	2m 58s
memory_sanitizer	18m 42s
minimal	2m 44s
no-deprecated	5m 8s
non-caching	12m 23s
no-shared-macos (macos-13)	6m 41s
no-shared-ubuntu	4m 39s
no-shared-macos (macos-14)	4m 22s
self-hosted	0s
threads_sanitizer	4m 59s
external-test-cf-quiche	3m 48s
external-tests	10m 29s
external-test-pyca (1.51.0, 3.9)	5m 30s
legacy	4m 37s
no-legacy	5m 40s
out-of-readonly-source-and-install-macos (macos-13)	13m 14s
out-of-readonly-source-and-install-ubuntu	7m 46s
out-of-readonly-source-and-install-macos (macos-14)	6m 36s
	2h 38m 33s

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