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"
```
The source included way more files than it really needed.
This commit limits it to exactly those it needs.
This also makes sure that no rebuild is necessary when any Nix
file changes, in particular useful when we reformat the Nix files.
We shouldn't try compiling to aarch64-unknown-linux-gnu from
aarch64-unknown-linux-gnu, because that tends to confuse things and is
not representative of actual cross compilation usage.
Sanitizers don't seem to be present on aarch64-darwin/macOS 12 (Monterey), so they are removed from the aarch64-darwin tests.
Switching from nativeBuildInputs to buildInputs and adding cc to the deps list caused some strange error messages to go away.
On macOS, /tmp is a symlink to /private/tmp. When performing cd /tmp, and checking cwd - it won't match since it follows the symlink.
This caused test breakage on macOS but not Linux. Instead, use a folder which is not a symlink, and consistent across Linux and macOS.
