To avoid confusion and spurious newlines in the output.
Found by searching for a backslash followed by a newline, some
indentation, and then the end of multi-line string marker, using the
following extended regex:
```regex
\\\n +'';
```
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>
In preparation for the deprecation of `stdenv.isX`.
These shorthands are not conducive to cross-compilation because they
hide the platforms.
Darwin might get cross-compilation for which the continued usage of `stdenv.isDarwin` will get in the way
One example of why this is bad and especially affects compiler packages
https://www.github.com/NixOS/nixpkgs/pull/343059
There are too many files to go through manually but a treewide should
get users thinking when they see a `hostPlatform.isX` in a place where it
doesn't make sense.
```
fd --type f "\.nix" | xargs sd --fixed-strings "stdenv.is" "stdenv.hostPlatform.is"
fd --type f "\.nix" | xargs sd --fixed-strings "stdenv'.is" "stdenv'.hostPlatform.is"
fd --type f "\.nix" | xargs sd --fixed-strings "clangStdenv.is" "clangStdenv.hostPlatform.is"
fd --type f "\.nix" | xargs sd --fixed-strings "gccStdenv.is" "gccStdenv.hostPlatform.is"
fd --type f "\.nix" | xargs sd --fixed-strings "stdenvNoCC.is" "stdenvNoCC.hostPlatform.is"
fd --type f "\.nix" | xargs sd --fixed-strings "inherit (stdenv) is" "inherit (stdenv.hostPlatform) is"
fd --type f "\.nix" | xargs sd --fixed-strings "buildStdenv.is" "buildStdenv.hostPlatform.is"
fd --type f "\.nix" | xargs sd --fixed-strings "effectiveStdenv.is" "effectiveStdenv.hostPlatform.is"
fd --type f "\.nix" | xargs sd --fixed-strings "originalStdenv.is" "originalStdenv.hostPlatform.is"
```
