Bug#914558: petsc: hardcodes path to sh depending on build host

[Drew Parsons]

Source: petsc
Followup-For: Bug #914558

This is the first time I've heard of src-merge. Nice if it helps
manage containers.  But irreponsible to introduce it this late into
the Buster release cycle.

src-merge means the system is changed.  You build your package for a
given system, you expect it run on that system, you don't expect it to
run on a different system, or on an older system.  It's the same as a
library ABI or libc upgrade. All affected packages need to be rebuilt
to ensure the archive is running consistently. 

As far as I'm concerned this is not a petsc bug. You did the right
thing asking for a binNMU, thanks for that.

Drew