how to get base /usr/bin/env in a package?

[ng0%n0.is@localhost]

At build time a package I'm working on is replacing
@xENV@ python2.7
with the result of which env, resulting in
/path/to/env python2.7

in pkgsrc this results in:

ERROR: /usr/pkg/bin/gnunet-qr:  #!/usr/work/wip/gnunet/work/.tools/bin/env python2.7
ERROR: *** The above files still have references to the build directory.
ERROR:     This is possibly an error that should be fixed by unwrapping
ERROR:     the files or adding missing tools to the package makefile!
*** Error code 1

I've tried to SUBST_SED, but it always stays at a wrong path.

Is there a way to achieve this without changing the upstream Makefile?