treewide: gate command execution for `installShellCompletion` behind `buildPlatform.canExecute hostPlatform`

This treewide conditions execution of the built applications for the purpose of generating shell completions behind `stdenv.buildPlatform.canExecute stdenv.hostPlatform`, which helps cross. This is a common issue I spot during review, let's prevent copy-paste errors by fixing it treewide.

Candidates were located with:

```shell
rg 'installShellCompletion' -l -tnix | xargs grep -F 'stdenv.buildPlatform.canExecute stdenv.hostPlatform' -L
```

Then I migrated the obvious cases which would not require a rebuild, as a means of testing.
This diff was not scripted. Should be zero rebuilds.

<details>
<summary>
Alternatives
</summary>

Alternatively I could have use this pattern:

```nix
postInstall = lib.optionalString (stdenv.hostPlatform.emulatorAvailable buildPackages) (
let
    emulator = stdenv.hostPlatform.emulator buildPackages;
in
''
    installShellCompletion --cmd foobar \
    --bash <(${emulator} $out/bin/foobar completion bash) \
    --fish <(${emulator} $out/bin/foobar completion fish) \
    --zsh <(${emulator} $out/bin/foobar completion zsh)
''
);
```

but that would cause rebuilds and will also require testing.

---

An other alternative is to use the binary from the corresponding package in `buildPackages`.
This however would also cause rebuilds and will also require testing.

</details>

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