vmTools.runInLinuxVM: create fd symlinks in /dev

These are usually set up by systemd, but there's no systemd in the
VM.  /dev/fd is required for <(...) syntax in bash, and I'm sure lots
of things depend on the stdin/stdout/stderr links as well.

pkgs/build-support/vm/default.nix

@@ -90,6 +90,10 @@ rec {
     done
 
     mount -t devtmpfs devtmpfs /dev
+    ln -s /proc/self/fd /dev/fd
+    ln -s /proc/self/fd/0 /dev/stdin
+    ln -s /proc/self/fd/1 /dev/stdout
+    ln -s /proc/self/fd/2 /dev/stderr
 
     ifconfig lo up