vmTools.runInLinuxVM: do not run postHook on the host

Since 26eba25577 ("runInLinuxVM: re-add sourcing of stdenv &
.attrs.sh"), the host side of runInLinuxVM sources $stdenv/setup so that
preVM can use $out. That also runs postHook on the host, but postHook is
meant for the build inside the VM. runInLinuxImage sets PATH to the
guest's /usr/bin:/bin in postHook, so releaseTools.debBuild's
propagateImageName then failed on the host with
"mkdir: command not found".

Fixes #433008

Assisted-by: Claude Code (claude-opus-5-5)

pkgs/build-support/vm/default.nix

@@ -293,6 +293,10 @@ let
         ${coreutils}/bin/cp $NIX_ATTRS_JSON_FILE $NIX_ATTRS_SH_FILE xchg
         source "$NIX_ATTRS_SH_FILE"
       fi
+      # The setup script is sourced here only so that `preVM` sees `$out` and
+      # friends. `postHook` belongs to the build inside the VM, where it may
+      # rely on the guest's tools (see `runInLinuxImage`), so skip it here.
+      postHook=
       source $stdenv/setup
 
       eval "$preVM"