Support non-root building path

[txabman42]

Coming from #120 (comment)

Let's move demo/basic/main.go to demo/basic/cmd/main.go and execute go build -a cmd/ from basic folder. This would work without problem, however, when instrumenting it with otel (../../otel go build -a cmd/), it fails:

Error logs

# cmd/trace
net/http.OtelBeforeTrampoline_ServeHTTP802697909: relocation target github.com/open-telemetry/opentelemetry-go-compile-instrumentation/pkg/instrumentation/nethttp.BeforeServeHTTP not defined
Error:
[0] failed to run command "/usr/local/go/pkg/tool/darwin_arm64/link" with args: [-o $WORK/b661/exe/a.out -importcfg $WORK/b661/importcfg.link -X=cmd/internal/objabi.buildID=d3PxNBJKoXj5HhChQTjx/PqTyxeY2x3r8w-sZE2K0/JDzRK1jzYRrptT4B3Kcy/d3PxNBJKoXj5HhChQTjx -buildmode=pie -buildid=d3PxNBJKoXj5HhChQTjx/PqTyxeY2x3r8w-sZE2K0/JDzRK1jzYRrptT4B3Kcy/d3PxNBJKoXj5HhChQTjx -extld=clang $WORK/b661/_pkg_.a]: exit status 2

Same would happen for https://github.com/VictoriaMetrics/VictoriaMetrics --> otel go build -a ./app/vmctl

This is mainly because otel.runtime.go is not generated in the target folder, but in the root one. Linker wont be able to resolve linkname directives.

It could be also interesting to test the instrumentation of a library.