treewide: replace stdenv.is with stdenv.hostPlatform.is

[Artturin]

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I'll reset to master and rerun the commands before merge.
Further replacements can be done in the PR where the warnings are added.
The commits can be added to .git-blame-ignore-revs in a separate PR

treewide: replace stdenv.is with stdenv.hostPlatform.is

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
#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  "buildStdenv.is" "buildStdenv.hostPlatform.is"
fd --type f "\.nix" | xargs sd  "effectiveStdenv.is" "effectiveStdenv.hostPlatform.is"
fd --type f "\.nix" | xargs sd  "originalStdenv.is" "originalStdenv.hostPlatform.is"

The rebuilds are due to src ./. or similar

https://gist.github.com/Artturin/4cc1e73fc612cb7a79459a9bb6bc27a3