build gets totally stuck after a broken download

[Dima Pasechnik]

Building the latest rc.
After a failure (network issues) to get an tarball for an spkg, I am
now seeing the following quite annoying bug:


$ make numpy # (or just make)
...
make[1]: Entering directory
'/home/scratch/scratch2/dimpase/sage/sage/build/make'
make --no-print-directory numpy-SAGE_VENV-no-deps
[numpy-1.21.4] cat: /build/pkgs/numpy/type: No such file or directory
[numpy-1.21.4] Error: Installing old-style SPKGs is no longer supported
make[2]: *** [Makefile:2793: numpy-SAGE_VENV-no-deps] Error 1
make[1]: *** [Makefile:2793:
/home/scratch/scratch2/dimpase/sage/sage/local/var/lib/sage/venv-python3.9/var/lib/sage/installed/numpy-1.21.4]
Error 2
make[1]: Leaving directory '/home/scratch/scratch2/dimpase/sage/sage/build/make'

...

There is no tarball for numpy in upstream/, and, as you see above,
the prefix for `build/pkgs/numpy/type` is wiped out, so the package
typecheck fails with
a silly error. It's unclear to me how to recover at this point.

(this is on Fedora 34, in case)