You might need --link, e.g.:
# inside the cloned pyffi folder
$ raco pkg install --link
(note: no need to specify "pyffi" again as it's inferred from the current directory name)
... although, I just tried that and, while the package did install, I saw some errors:
pyffi-doc/pyffi/scribblings/manual-pyffi.scrbl:9:21: cannot open module file
module path: pyffi
path: /Users/siddhartha/work/racket/pyffi/main.rkt
system error: no such file or directory; rkt_err=3
and
ffi-lib: could not load foreign library
path: libpython3.10.dylib
system error: dlopen(libpython3.10.dylib, 10): image not found
context...:
/Applications/Racket-Latest/collects/ffi/unsafe.rkt:131:0: get-ffi-lib
body of "/Users/siddhartha/work/racket/pyffi/pyffi-lib/pyffi/libpython.rkt"
...
raco pkg install: packages installed, although setup reported errors
I haven't followed this thread in detail so apologies if I've missed something above.