fix: pass slurm account to inference srun call

[lclanzi]

Summary

• The inference_execute rule runs anemoi-inference via a nested srun inside a squashfs-mount ... bash -c block, separate from snakemake's own job submission
• slurm_account (added in Expose SLURM account and GPUs from config #180) was only wired into snakemake's --default-resources, so it never reached this inner srun call. This meant that GPU inference jobs ran under the cluster's default account instead of the one configured in profile.default_resources.slurm_account
• Adds an account_flag param that renders --account=<slurm_account> when configured, or an empty string otherwise, and passes it to the inner srun call.

[frazane]

LGTM, thanks!