bug#37035: --with-graft tries to download source files and build them

[pkill9]

When running `guix build --dry-run --with-graft=mesa=mesa love`, Guix shows 
that it will try to download a bunch of source files for quite a few packages:

```
$ guix build --dry-run --with-graft=mesa=mesa love
The following derivations would be built:
   /gnu/store/0lfws1b85h6lkv09pp7q0439pd41qirj-love-11.1.drv
   /gnu/store/2spqmdn4s13r0z9d9hym25h4nfcl1r5g-sdl2-2.0.9.drv
   /gnu/store/aknpb0pgslx9jaidgvh8wrzl7j6nl3nn-fcitx-4.2.9.6.drv
   /gnu/store/8pk7gncsr1cfph8km8dhwk67q9d58c0k-libepoxy-1.5.3.drv
   /gnu/store/w0wxk5f2a44hksrbvwqxn1d6sx96hdyw-gtk+-3.24.9.drv
   /gnu/store/ydx7mg4bni0lfckw0r3x65icmfhr194k-xorg-server-1.20.5.drv
   /gnu/store/xnc6a1fjwlwfjpg0lwai3aq51rw5qwrc-extra-cmake-modules-5.55.0.drv
   /gnu/store/jp29lhdwhs3znbsbznxq9nd9zp5l8ds0-qtbase-5.11.3.drv
   /gnu/store/sblg1v3yxf41b89aaj4r47zcyhxs3az4-ibus-1.5.20.drv
   /gnu/store/a4ynsy7ras28jlgmrhfilkc0lnin21mw-libnotify-0.7.7.drv
   /gnu/store/d8gvjnrl584wc5pavppa3m3yvm3jaxpk-dconf-0.32.0.drv
76.4 MB would be downloaded:
   /gnu/store/4vl3vkvqv0hhr0rlzciq55l4j8c5lgdz-ibus-1.5.20.tar.gz
   /gnu/store/jpxqw86l1gxkfrp66h5mc747xf881hjm-gettext-0.19.8.1
   /gnu/store/9dgbqa3m412prwadpwg0ah7a3kv6fh9p-libnotify-0.7.7.tar.xz
   /gnu/store/aaha1wfv58b83fwp0fqg8b5nlpkw1bva-libmng-2.0.3
   /gnu/store/6qy0ni5msmg93a55wi5qmj96729c1fcf-vulkan-headers-1.1.112
   
/gnu/store/3a3j7rnd49fr6papzd2r2i8f96ayxi1v-qtbase-everywhere-src-5.11.3.tar.xz
   /gnu/store/3n7yhbfl1gik3n64rmy1574djpc1w6m5-gperf-3.1
   /gnu/store/wkfsm7mv8rjawdkzi8chf7qy8l5dppkm-xorg-server-1.20.5.tar.xz
   /gnu/store/7w7fq1iy0jkap5np4q00cw4cllr0lkj5-libdmx-1.1.4
   /gnu/store/cc39n3mx1nswgwx7p8mbx7apy4j1w8yy-xtrans-1.3.5
   /gnu/store/l1nxv1asf958h9ww4iijypyk31mj3d3x-gtk+-3.24.9.tar.xz
   /gnu/store/9rzjddyd621n26av5hb6zn55r372fhnv-libepoxy-1.5.3.tar.xz
```