stdenv: Add hack to fix cmake canExecute cross-compilation

Fixes `pkgsCross.musl64.llvmPackages_16.clang.cc` on `x86_64-linux`,
which used to fail with `/bin/sh: clang-tblgen: not found`.

Same hack is used in other projects:
https://github.com/search?q=%2FCMAKE_CROSSCOMPILING_EMULATOR.%2B%5C%2Fusr%5C%2Fbin%5C%2Fenv%2F+NOT+is%3Afork&type=code

Comment from https://github.com/vsrinivas/fuchsia/blob/30435a9d0f0b67c94e3c70760b522c9f7fbbd6be/build/cmake/HostLinuxToolchain.cmake#L64

> Required to run host Linux executables during the build itself.
> An example would be https://gitub.com/KhronosGroup/Vulkan-Loader and
> its "asm_offset" program.
>
> NOTE: Alternatives have been tried unsuccessfully, i.e.:
>
>  With $(set CMAKE_CROSSCOMPILING_EMULATOR), the build fails because
>  the CMake ninja/Make script tries to find the executable in the current
>  path, as in:
>
>    [3/16] Generating gen_defines.asm
>    FAILED: loader/gen_defines.asm
>    cd /tmp/cc/build-Vulkan-Loader/loader && asm_offset GAS
>    /bin/sh: asm_offset: command not found
>    ninja: build stopped: subcommand failed.
>
> With $(set CMAKE_CROSSCOMPILING_EMULATOR ""), the build fails because
> the shell cannot find the "" program as in:
>
>    [3/16] Generating gen_defines.asm
>    FAILED: loader/gen_defines.asm
>    cd /tmp/cc/build-Vulkan-Loader/loader && "" /tmp/cc/build-Vulkan-Loader/loader/asm_offset GAS
>    /bin/sh: : command not found
>    ninja: build stopped: subcommand failed.
>
> It seems that the root of the problem comes from how the CMake function
> cmCustomCommandGenerator::GetArgc0Location() computes the target
> executable's location. At this point it's unclear whether this is a CMake
> bug or a feature.

Risicle discovered this hack.

Co-authored-by: Robert Scott <code@humanleg.org.uk>

pkgs/stdenv/generic/make-derivation.nix

@@ -377,6 +377,8 @@ else let
           "-DCMAKE_HOST_SYSTEM_PROCESSOR=${stdenv.buildPlatform.uname.processor}"
         ] ++ lib.optionals (stdenv.buildPlatform.uname.release != null) [
           "-DCMAKE_HOST_SYSTEM_VERSION=${stdenv.buildPlatform.uname.release}"
+        ] ++ lib.optionals (stdenv.buildPlatform.canExecute stdenv.hostPlatform) [
+          "-DCMAKE_CROSSCOMPILING_EMULATOR=env"
         ]);
 
       mesonFlags =