rktboot error on aarch64-linux

According to Guix CI, bootstrapping Raket's Chez Scheme fork fails with an error from string-append. Here's the tail of the build log:

starting phase `build'
Assuming current directory has Chez Scheme sources
string-append: contract violation
  expected: string?
  given: #f
  argument position: 1st
  other arguments...:
   ".def"
  context...:
   /tmp/guix-build-racket-bootstrap-chez-bootfiles-8.2.drv-0/racket-minimal-8.2-checkout/racket/src/ChezScheme/rktboot/machine-def.rkt:6:0: open-file-with-machine.def-redirect
command "/gnu/store/1m10cjmip18isdxf0bljq733brq2gg9p-racket-minimal-bc-3m-8.2/bin/racket" "rktboot/main.rkt" "--dest" "/gnu/store/xlsnnphcqn1xxp2yqr519jh9sv0ai9xw-racket-bootstrap-chez-bootfiles-8.2" failed with status 1
builder for `/gnu/store/xg8sa6qssr46n5n887bzb5l3cl78fy52-racket-bootstrap-chez-bootfiles-8.2.drv' failed with exit code 1
@ build-failed /gnu/store/xg8sa6qssr46n5n887bzb5l3cl78fy52-racket-bootstrap-chez-bootfiles-8.2.drv - 1 builder for `/gnu/store/xg8sa6qssr46n5n887bzb5l3cl78fy52-racket-bootstrap-chez-bootfiles-8.2.drv' failed with exit code 1

The offending string-append seems to be here: https://github.com/racket/racket/blob/4f0e1703979d704c4f91b009945c324450acec7a/racket/src/ChezScheme/rktboot/machine-def.rkt#L10

I don't know if target-machine being #f is indicative of a broader problem inferring the target machine or not. A further complication is that I don't have an aarch64-linux setup myself, though I could try to get something working via QEMU.

I hope this problem isn't specific to Guix, but the build is driven by this Guile code, which I can (hopefully) explain if necessary: https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/racket.scm

Start with racket/src/ChezScheme/rktboot/machine-def.rkt, especially open-file-with-machine.def-redirect, and review the linked Guix build log. Compare how target-machine is inferred on aarch64-linux with the package definition in Guix's gnu/packages/racket.scm. Done means the Racket bootstrap build completes on aarch64-linux without the string-append contract violation.