[cmd/telemetrygen] telemetrygen build takes too long and doesn't use caching

[atoulme]

Component(s)

cmd/telemetrygen

Describe the issue you're reporting

The telemetrygen github actions build consists in cross-compiling and creating Docker images for different archs. The issue is that this takes a long time because no caching is present.

It might prove a problem as we get throttled or rejected if we have too many builds, and the build itself is long (> 12 minutes).

Example:
https://github.com/open-telemetry/opentelemetry-collector-contrib/actions/runs/7732736868/job/21083320048?pr=30518

[github-actions]

Pinging code owners:

• cmd/telemetrygen: @mx-psi @codeboten

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