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"
```
Eliminate uses of `config.cudaSupport or false` and alike, since the
option is now declared in config.nix with a default value
fd .nix -t f -x sed 's/config\.cudaSupport or false, cudaPackages [?] [{][}]/config.cudaSupport, cudaPackages ? { }/' '{}' -i
fd .nix -t f -x sed 's/config\.cudaSupport or false/config.cudaSupport/' '{}' -i
fd .nix -t f -x sed 's/cudaSupport = pkgs.config.cudaSupport/inherit (pkgs.config) cudaSupport/' '{}' -i
fd .nix -t f -x sed 's/cudaSupport = config.cudaSupport/inherit (config) cudaSupport/' '{}' -i
'cudaSupport ? false' -> 'cudaSupport ? config.cudaSupport or false' to respect global defaults
Packages expressions that take `cudaSupport ? false` are likely to
ignore `config.cudaSupport`. Instead, we want them to make `cudaSupport`
a required argument, or to explicitly refer to `config`
