Allow building via Flake on `aarch64-linux`

Desired Behavior

If I run nix build .#dune-static (or .#dune) on Ubuntu 20.04 on ARM I would want Dune to build. It currently fails because it can't find a matching C compiler.

Example

nix build .#dune-static
building '/nix/store/1gp9cay6pccy1208vm2qik2ppcn8824j-ocaml+flambda-x86_64-unknown-linux-musl-5.4.1.drv'...
error: Cannot build '/nix/store/1gp9cay6pccy1208vm2qik2ppcn8824j-ocaml+flambda-x86_64-unknown-linux-musl-5.4.1.drv'.
       Reason: builder failed with exit code 1.
       Output paths:
         /nix/store/mxrj4fgpa6ifzbqh7pbrsbm3jml046vc-ocaml+flambda-x86_64-unknown-linux-musl-5.4.1
       Last 25 log lines:
       > checking whether stripping libraries is possible... yes
       > checking if libtool supports shared libraries... yes
       > checking whether to build shared libraries... yes
       > checking whether to build static libraries... yes
       > checking if x86_64-unknown-linux-musl-gcc generates a .note.GNU-stack section... yes
       > checking if x86_64-unknown-linux-musl-gcc generates .size and .type asm directives... yes
       > checking how to run the C preprocessor... x86_64-unknown-linux-musl-gcc -E
       > checking C compiler vendor... gcc-15-2
       > checking whether target executables can be run in the build... no
       > checking whether #! works in shell scripts... yes
       > checking whether the C compiler accepts -Wold-style-declaration... yes
       > checking whether the C compiler accepts -Wimplicit-fallthrough=5... yes
       > checking for flexdll sources... checking for flexdll.h... no
       > configure: detecting the C toolchain for build
       > checking for aarch64-unknown-linux-musl-gcc... no
       > checking for gcc... no
       > checking for aarch64-unknown-linux-musl-cc... no
       > checking for cc... no
       > checking for aarch64-unknown-linux-musl-cl.exe... no
       > checking for cl.exe... no
       > checking for aarch64-unknown-linux-musl-clang... no
       > checking for clang... no
       > configure: error: in `/build/source':
       > configure: error: no acceptable C compiler found in $PATH
       > See `config.log' for more details
       For full logs, run:
         nix log /nix/store/1gp9cay6pccy1208vm2qik2ppcn8824j-ocaml+flambda-x86_64-unknown-linux-musl-5.4.1.drv
error: Cannot build '/nix/store/r154ay8a9sp7sknmmad690y58si51inx-dune-x86_64-unknown-linux-musl-3.23.1.drv'.
       Reason: 1 dependency failed.
       Output paths:
         /nix/store/xynwzl8mi5l3b860y82vp5zgyz0f57mr-dune-x86_64-unknown-linux-musl-3.23.1
error: Cannot build '/nix/store/hgnc08jsdfm6489w7imxpdchaslap7jr-ocaml5.4.1-findlib-x86_64-unknown-linux-musl-1.9.8.drv'.
       Reason: 1 dependency failed.
       Output paths:
         /nix/store/cvdml64cmlcjzd17iyfwx95j0i09lcvb-ocaml5.4.1-findlib-x86_64-unknown-linux-musl-1.9.8
error: Build failed due to failed dependency

Start by reproducing nix build .#dune-static or .#dune on Ubuntu 20.04 running on ARM and inspect the reported Nix build log. Trace the flake entry and toolchain configuration involved in selecting the C compiler for aarch64-linux; done means both targets build successfully on that platform.