Use pkg-config to construct CGO_LDFLAGS #879

Job	Run time
Build libbpf in Docker (aarch64, emulated)	3m 27s
Build libbpf in Docker (x86_64)	26s
Build ebpf_exporter in Docker (x86_64, built-in libbpf)	6h 0m 16s
Build ebpf_exporter (x86_64, dynamically linked, built-in libbpf)	26s
Build ebpf_exporter in Docker (aarch64, emulated, built-in libbpf)	11m 37s
Build ebpf_exporter (x86_64, statically linked, built-in libbpf)	38s
Build examples (x86_64, built-in libbpf)	32s
Build tracing demos (x86_64)	39s
Run clang-format check	11s
Lint ebpf_exporter (x86_64, built-in libbpf)	25s
Run markdown-links-check	27s
Run jsonschema checks	10s
Test ebpf_exporter (x86_64, built-in libbpf)	55s
Publish Docker images (x86_64, aarch64)	2m 19s
Build ebpf_exporter (x86_64, statically linked, system libbpf)	33s
Build ebpf_exporter (x86_64, dynamically linked, system libbpf)	29s
Build examples (x86_64, system libbpf)	22s
Test ebpf_exporter (x86_64, system libbpf)	56s
Check examples	0s
Release archive	0s
	6h 24m 48s

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