podman: fix cross compilation build

- See `Nixpkgs` issue 467994 for detailed build logs showing the
  failure.

- `Podman` will use its vendored version of `go-md2man` when the
  `GOMD2MAN` environment variable is unset.

- When cross compiling from `x86_64-linux` to `aarch64-linux`, this
  results in the tool being compiled with the `aarch64-linux` compiler
  using `x86_64-linux` options. The compiler arch is wrong, so the build
  fails.

- To circumvent this use `Nixpkgs` version of `go-md2man`.

- The current vendored version matches `Nixpkgs` version (v2.0.7):
  https://github.com/containers/podman/commit/7e14a0340922a03781f9f6238a914b04a749573f

- The reason for the vendoring is detailed in this commit:
  https://github.com/containers/podman/commit/dfec510b41b536d4a3afc810cab335d1d0feb3e0

pkgs/by-name/po/podman/package.nix

@@ -5,6 +5,7 @@
   pkg-config,
   installShellFiles,
   buildGoModule,
+  buildPackages,
   gpgme,
   lvm2,
   btrfs-progs,
@@ -12,7 +13,6 @@
   libseccomp,
   libselinux,
   systemdMinimal,
-  go-md2man,
   nixosTests,
   python3,
   makeBinaryWrapper,
@@ -71,7 +71,6 @@ buildGoModule (finalAttrs: {
 
   nativeBuildInputs = [
     pkg-config
-    go-md2man
     installShellFiles
     makeBinaryWrapper
     python3
@@ -90,6 +89,7 @@ buildGoModule (finalAttrs: {
   env = {
     HELPER_BINARIES_DIR = "${placeholder "out"}/libexec/podman"; # used in buildPhase & installPhase
     PREFIX = "${placeholder "out"}";
+    GOMD2MAN = "${buildPackages.go-md2man}/bin/go-md2man";
   };
 
   buildPhase = ''