Fix race condition in Expiry Map

[mariomac]

Fixes this race condition:

WARNING: DATA RACE
Write at 0x00c00052a300 by goroutine 89:
  runtime.mapassign_faststr()
      /opt/hostedtoolcache/go/1.22.4/x64/src/runtime/map_faststr.go:203 +0x0
  github.com/grafana/beyla/pkg/internal/export/expire.(*ExpiryMap[go.shape.*uint8]).GetOrCreate()
      /home/runner/work/beyla/beyla/pkg/internal/export/expire/expiry_map.go:58 +0x459

(....)

Previous read at 0x00c00052a300 by goroutine 107:
  github.com/grafana/beyla/pkg/internal/export/expire.(*ExpiryMap[go.shape.*uint8]).All()
      /home/runner/work/beyla/beyla/pkg/internal/export/expire/expiry_map.go:92 +0xc4

(....)