apps/cmp: improve -reqin option to read fallback public key from firs… #951

Job	Run time
compiler (gcc-7, ubuntu-20.04)	7m 50s
compiler (gcc-8, ubuntu-20.04)	8m 49s
compiler (gcc-9, ubuntu-20.04)	7m 37s
compiler (gcc-10, ubuntu-20.04)	7m 39s
compiler (gcc-11, ubuntu-22.04)	7m 39s
compiler (gcc-12, ubuntu-22.04)	9m 4s
compiler (gcc-13, ubuntu-22.04, ubuntu-toolchain-r/test)	8m 26s
compiler (clang-6.0, ubuntu-20.04)	8m 59s
compiler (clang-7, ubuntu-20.04)	7m 45s
compiler (clang-8, ubuntu-20.04)	8m 14s
compiler (clang-9, ubuntu-20.04)	10m 6s
compiler (clang-10, ubuntu-20.04)	7m 44s
compiler (clang-11, ubuntu-20.04)	9m 36s
compiler (clang-12, ubuntu-20.04)	7m 34s
compiler (clang-13, ubuntu-22.04)	8m 12s
compiler (clang-14, ubuntu-22.04)	7m 53s
compiler (clang-15, ubuntu-22.04, jammy)	8m 6s
compiler (clang-16, ubuntu-22.04, jammy)	10m 44s
	2h 31m 57s

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