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, 2 insertions, 2 deletions

diff --git a/pkgs/servers/monitoring/grafana-agent/default.nix b/pkgs/servers/monitoring/grafana-agent/default.nix
index fd1307c10f61..c45ea47bac92 100644
--- a/pkgs/servers/monitoring/grafana-agent/default.nix
+++ b/pkgs/servers/monitoring/grafana-agent/default.nix
@@ -79,12 +79,12 @@ buildGoModule rec {
 
   # uses go-systemd, which uses libsystemd headers
   # https://github.com/coreos/go-systemd/issues/351
-  env.NIX_CFLAGS_COMPILE = toString (lib.optionals stdenv.isLinux [ "-I${lib.getDev systemd}/include" ]);
+  env.NIX_CFLAGS_COMPILE = toString (lib.optionals stdenv.hostPlatform.isLinux [ "-I${lib.getDev systemd}/include" ]);
 
   # go-systemd uses libsystemd under the hood, which does dlopen(libsystemd) at
   # runtime.
   # Add to RUNPATH so it can be found.
-  postFixup = lib.optionalString stdenv.isLinux ''
+  postFixup = lib.optionalString stdenv.hostPlatform.isLinux ''
     patchelf \
       --set-rpath "${lib.makeLibraryPath [ (lib.getLib systemd) ]}:$(patchelf --print-rpath $out/bin/grafana-agent)" \
       $out/bin/grafana-agent