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
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"
```

1 files changed, 1 insertions, 1 deletions

diff --git a/pkgs/development/python-modules/pymemcache/default.nix b/pkgs/development/python-modules/pymemcache/default.nix
index c80f512e74982..6f570f8d83065 100644
--- a/pkgs/development/python-modules/pymemcache/default.nix
+++ b/pkgs/development/python-modules/pymemcache/default.nix
@@ -40,7 +40,7 @@ buildPythonPackage rec {
     sed -i "/--cov/d" setup.cfg
   '';
 
-  disabledTests = lib.optionals stdenv.is32bit [
+  disabledTests = lib.optionals stdenv.hostPlatform.is32bit [
     # test_compressed_complex is broken on 32-bit platforms
     # this can be removed on the next version bump
     # see also https://github.com/pinterest/pymemcache/pull/480