build fails on AArch64, Fedora 33

[jw@cn05 rstan]$ ./sh_b.sh
Submodule 'StanHeaders/inst/include/mathlib' (https://github.com/stan-dev/math.git) registered for path 'StanHeaders/inst/include/mathlib'
Submodule 'stan' (https://github.com/stan-dev/stan.git) registered for path 'StanHeaders/inst/include/upstream'
Cloning into '/home/jw/stan/rstan/StanHeaders/inst/include/mathlib'...
Cloning into '/home/jw/stan/rstan/StanHeaders/inst/include/upstream'...
Submodule path 'StanHeaders/inst/include/mathlib': checked out 'da6d954ad871356075eb4001d121ff1c6bbe6c99'
Submodule path 'StanHeaders/inst/include/upstream': checked out '262797e4c2e156213fc921c8363efbf489ca6ee2'
+da6d954ad871356075eb4001d121ff1c6bbe6c99 StanHeaders/inst/include/mathlib (v4.0.1-430-gda6d954ad8)
+262797e4c2e156213fc921c8363efbf489ca6ee2 StanHeaders/inst/include/upstream (v2.26.0-92-g262797e4c)

• checking for file 'StanHeaders/DESCRIPTION' ... OK
  cp: cannot stat 'StanHeaders/inst/include/libsundials': No such file or directory
  ERROR
  copying to build directory failed
  find: 'StanHeaders*.tar.gz': No such file or directory
  tar: Old option 'f' requires an argument.
  Try 'tar --help' or 'tar --usage' for more information.
  ERROR: stan/version.hpp is not found in StanHeaders pkg
  [jw@cn05 rstan]$

Reproduce the failure with ./sh_b.sh on the reported AArch64 Fedora 33 environment. Start by tracing the StanHeaders/inst/include copy step and the missing libsundials path; done means the package builds successfully and the generated StanHeaders archive contains stan/version.hpp.