How to add /bin/sh to the build environment

[Foo Chuan Wei]

I am trying to package the Standard ML of New Jersey (SML/NJ) compiler
in Guix. My current understanding is that its installation process
relies on the existence of /bin/sh in a fundamental way. Is there a way
to add /bin/sh to the build environment?

In the build phase of the package definition, I tried something like
this:

    (mkdir-p "/bin")
    (symlink (which "sh") "/bin/sh")

However, `mkdir-p` always fails with:

    ice-9/boot-9.scm:1669:16: In procedure raise-exception:
    In procedure mkdir: Permission denied