Add external directory for Conan recipe's exports #3024

Job	Run time
dependencies (linux, gcc, Debug)	2m 14s
dependencies (linux, gcc, Release)	1m 39s
dependencies (linux, clang, Debug)	1m 54s
dependencies (linux, clang, Release)	1m 33s
coverage (linux, gcc, Debug)	22m 23s
test (linux, gcc, Debug)	12m 36s
test (linux, gcc, Debug, -Dunity=ON)	7m 28s
test (linux, gcc, Release)	14m 39s
test (linux, gcc, Release, -Dunity=ON)	8m 52s
test (linux, clang, Debug)	10m 3s
test (linux, clang, Debug, -Dunity=ON)	6m 5s
test (linux, clang, Release)	12m 55s
test (linux, clang, Release, -Dunity=ON)	10m 22s
	1h 52m 43s

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