gnu: pigx-bsseq: Fix Python module capture.

The build system expects all Python modules to be on PYTHONPATH.

* gnu/packages/bioinformatics.scm (pigx-bsseq)[arguments]: Set PYTHONPATH
variable in new build phase.

gnu/packages/bioinformatics.scm

@@ -11061,6 +11061,9 @@ in an easily configurable manner.")
          (add-before 'bootstrap 'autoreconf
            (lambda _
              (invoke "autoreconf" "-vif")))
+         (add-before 'configure 'set-PYTHONPATH
+           (lambda _
+             (setenv "PYTHONPATH" (getenv "GUIX_PYTHONPATH"))))
          (add-before 'check 'set-timezone
            ;; The readr package is picky about timezones.
            (lambda* (#:key inputs #:allow-other-keys)