SHELL leaking through to package build (breaks lang/tcl)

[David Brownlee <abs%absd.org@localhost>]

Should the logic shell be leaking through to packages for them to use
when building?

in lang/tcl

% make package SHELL=/usr/pkg/bin/zsh
> Generating pre-install file lists
Installing libtcl86.so to /var/obj/pkg/lang/tcl/work/.destdir/usr/pkg/lib/
/var/obj/pkg/lang/tcl/work/tcl8.6.6/unix/install-sh:272: command not found: cp
*** Error code 127

% make package SHELL=/bin/sh
[...]
=> Creating binary package
/opt/netbsd/pkgsrc/packages/NetBSD-7.1-x86_64/All/tcl-8.6.6nb2.tgz