Add per-target LIBBPF_SYS_LIBRARY_PATH env var to help cross-compilation #277

Job	Run time
Test on Alpine Linux x86_64 ()	29s
Test on Alpine Linux x86_64 (--no-default-features)	27s
Test libbpf-rs integration	28s
Check generated Rust bindings	38s
Test on Ubuntu amd64 (-F static,vendored)	1m 27s
Test on Ubuntu amd64 (--no-default-features)	38s
Test on Ubuntu amd64 ()	56s
Lint with clippy	46s
Test on Ubuntu arm64 (-F static,vendored)	1m 22s
Test on Ubuntu riscv64 (-F static,vendored)	1m 22s
Test on Ubuntu arm64 ()	1m 11s
	9m 44s

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