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"
```

pkgs/development/python-modules/aiohttp/default.nix

@@ -113,8 +113,8 @@ buildPythonPackage rec {
       # don't run benchmarks
       "test_import_time"
     ]
-    ++ lib.optionals stdenv.is32bit [ "test_cookiejar" ]
-    ++ lib.optionals stdenv.isDarwin [
+    ++ lib.optionals stdenv.hostPlatform.is32bit [ "test_cookiejar" ]
+    ++ lib.optionals stdenv.hostPlatform.isDarwin [
       "test_addresses" # https://github.com/aio-libs/aiohttp/issues/3572, remove >= v4.0.0
       "test_close"
     ];
@@ -129,7 +129,7 @@ buildPythonPackage rec {
 
       export HOME=$(mktemp -d)
     ''
-    + lib.optionalString stdenv.isDarwin ''
+    + lib.optionalString stdenv.hostPlatform.isDarwin ''
       # Work around "OSError: AF_UNIX path too long"
       export TMPDIR="/tmp"
     '';