Fixes <https://issues.guix.gnu.org/46212>.
Reported by Christopher Baines <mail@cbaines.net>.
Previously, the nar size returned by 'guix substitute' would be read as
an 'int'; thus, values above 2^31 - 1 would be read and then stored as
negative integers in the database.
Regression introduced in 9dfa20a22a.
* nix/libstore/build.cc (SubstitutionGoal::finished): Use templatized
'string2Int' instead of 'std::atoi' to get an 'unsigned long long',
which is the type of 'hash.second'.
* tests/store.scm ("substitute and large size"): New test.
