source nix-profile.sh only once

[juhp]

This is basically #12805 (nix-daemon.sh) for profile.d/nix.sh.

See #5950 (comment)

• Add a check to ensure the script executes only once per shell.

This makes bash behave the same way as fish,
which checks __ETC_PROFILE_NIX_SOURCED in both nix.fish and nix-daemon.fish.

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[juhp]

Thanks @convoliution - your approach in #15005 may be better perhaps - I haven't tested.
So I think it is better to review that first.

(Currently in the meantime I did apply the patch here to fedora, but I could drop sooner or later if it causes trouble and particularly if there is a better solution.)

[convoliution]

@juhp whatever the community prefers! I'm not too up-to-date on the Nix user base's needs, just felt the effects of #13255 enough to sink some time into what I thought might be a passable solution