integration: fix rootless tests

[crazy-max]

fixes #4100

Using an env var to set the otel socket path for the integration tests in #4078 requires to preserve the environment variables when using sudo for rootless mode but we can't use -E when we run a login shell (-i).

Specifying BUILDKIT_TRACE_SOCKET in buildkitArgs for oci and containerd workers in integration tests is tedious as tmpdir is not yet defined so I switched from an env var to a new otel-socket-path flag for the buildkitd command. I took the opportunity to also set a new section in the TOML config if that makes sense. Let me know if you prefer a new PR for this or if the flag is enough.

[crazy-max]

https://github.com/moby/buildkit/actions/runs/5751007434/job/15588925815?pr=4101#step:7:365

 #43 exporting to GitHub Actions Cache
#43 preparing build cache for export
#43 preparing build cache for export 2.1s done
#43 ERROR: Get "https://acghubeus1.actions.githubusercontent.com/VSeSJsGRiRCTiu0PRaYdcZnubUW6TSVddo0DTZpqG1wDqsIbXb/_apis/artifactcache/cache?keys=buildkit-blob-1-sha256%3A0b1a15617222e5f0617d075bcb63178f87a8a0f00ac47d53889e1cb3a92f673e&version=693bb7016429d80366022f036f84856888c9f13e00145f5f6f4dce303a38d6f2": tls: failed to verify certificate: x509: certificate signed by unknown authority

First time I see this one. Looks like an issue in GitHub infra.