bug#33234: Guix (weather): there can be only one

[Tobias Geerinckx-Rice]

Guix,

Two simultaneous ‘guix weather’s on the same machine will step on 
each others' toes and die.

They were running in two separate repositories, both vanilla 
sv:guix, one at v0.15.0, the other on master (7a7c91a).

Both aborted like this:

λ ./pre-inst-env guix weather 
--substitute-urls=https://munich.tobias.gr
 computing 7,850 package derivations for x86_64-linux...
 looking for 8,167 store items on https://munich.tobias.gr...
 updating substitutes from 'https://munich.tobias.gr'... 
 19.7%[my \n]
 guix weather: error: rename-file: No such file or directory

The second one displayed a different percentage and package count 
but was otherwise identical.

Kind regards,

T G-R