bug#31187: core-updates: url-fetch/tarbomb, url-fetch/zipbomb fail with "unbound variable: invoke"

[Eric Bavier]

In commit 20927c9331b493eaf94211ad9f8a5055e11b4588
url-fetch/tarbomb and url-fetch/zipbomb in guix/download.scm were
switched to use 'invoke' instead of 'system*'.  On core-updates this
leads for me to an error when attempting to build the source for
packages that use these fetch methods.  For example, font-text-gyre:

$ ./pre-inst-env guix build -S font-tex-gyre
The following derivation will be built:
   /gnu/store/clxzrqzqbn182nrnkpabd8f4kqfw5bna-tg-2.005otf.zip.drv
@ build-started /gnu/store/clxzrqzqbn182nrnkpabd8f4kqfw5bna-tg-2.005otf.zip.drv 
- x86_64-linux 
/var/log/guix/drvs/cl//xzrqzqbn182nrnkpabd8f4kqfw5bna-tg-2.005otf.zip.drv.bz2
Backtrace:
           3 (primitive-load "/gnu/store/c1r3fzw5mdh9hqydm2ri2rbdsib?")
In ice-9/eval.scm:
   196:27  2 (_ #f)
   223:20  1 (proc #<directory (guile-user) 7cc140>)
In unknown file:
           0 (%resolve-variable (7 . invoke) #<directory (guile-user?>)

ERROR: In procedure %resolve-variable:
Unbound variable: invoke
note: keeping build directory `/tmp/guix-build-tg-2.005otf.zip.drv-2'
builder for `/gnu/store/clxzrqzqbn182nrnkpabd8f4kqfw5bna-tg-2.005otf.zip.drv' 
failed with exit code 1
@ build-failed /gnu/store/clxzrqzqbn182nrnkpabd8f4kqfw5bna-tg-2.005otf.zip.drv 
- 1 builder for 
`/gnu/store/clxzrqzqbn182nrnkpabd8f4kqfw5bna-tg-2.005otf.zip.drv' failed with 
exit code 1
guix build: error: build failed: build of 
`/gnu/store/clxzrqzqbn182nrnkpabd8f4kqfw5bna-tg-2.005otf.zip.drv' failed
$

If I revert that commit it succeeds.  I suppose the (guix build utils)
module needs to be imported into the builder.

`~Eric

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