Set LD_LIBRARY_PATH and LIBRARY_PATH only if needed in CI #269

Job	Run time
build (libgccjit.so, master, --mini-tests)	5m 12s
build_system	19s
duplicates	3s
build (libgccjit.so, master, --std-tests)	5m 9s
build (libgccjit.so, master, --test-libcore)	12m 17s
build (libgccjit.so, master, --extended-rand-tests)	5m 57s
build (libgccjit.so, master, --extended-regex-example-tests)	6m 1s
build (libgccjit.so, master, --extended-regex-tests)	11m 17s
build (libgccjit.so, master, --test-successful-rustc --nb-parts 2 --current-part 0)	11m 14s
build (libgccjit.so, master, --test-successful-rustc --nb-parts 2 --current-part 1)	10m 49s
build (libgccjit.so, master, --projects)	17m 15s
build (libgccjit_without_int128.so, master-without-128bit-integers, --mini-tests)	4m 59s
build (libgccjit_without_int128.so, master-without-128bit-integers, --std-tests)	6m 42s
build (libgccjit_without_int128.so, master-without-128bit-integers, --test-libcore)	12m 47s
build (libgccjit_without_int128.so, master-without-128bit-integers, --extended-rand-tests)	6m 26s
build (libgccjit_without_int128.so, master-without-128bit-integers, --extended-regex-example-tests)	6m 7s
build (libgccjit_without_int128.so, master-without-128bit-integers, --extended-regex-tests)	10m 56s
build (libgccjit_without_int128.so, master-without-128bit-integers, --test-successful-rustc --nb-...	11m 9s
build (libgccjit_without_int128.so, master-without-128bit-integers, --test-successful-rustc --nb-...	11m 24s
build (libgccjit_without_int128.so, master-without-128bit-integers, --projects)	16m 48s
	2h 52m 51s

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