fetchzip: force UTF-8 compatibel locale to unpack non-ASCII symbols

musl and darwin support UTF-8 locales without any extras. As a result
unzip can unpack UTF-8 filenames there as is. But on glibc without
locale archive presence files get mangled as:

    deps/αβ -> deps/#U03b1#U03b2

This makes `fetchzip` fixed-output derivations unstable.

Tested this change to fail in `coq.src` which was generated in system
that mangles UTF-8 symbols:

    $ nix build -f. coq.src --rebuild -L
    source> trying https://github.com/coq/coq/archive/V8.15.2.zip
    source>   % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
    source>                                  Dload  Upload   Total   Spent    Left  Speed
    source>   0     0    0     0    0     0      0      0 --:--:--  0:00:01 --:--:--     0
    source> 100 8945k  100 8945k    0     0  1513k      0  0:00:05  0:00:05 --:--:-- 1989k
    source> unpacking source archive /build/V8.15.2.zip
    error: hash mismatch in fixed-output derivation '/nix/store/hrnyykm7wgw8vxisgq7hc2bg5gr0y6s8-source.drv':
             specified: sha256-h81nFqkuvZkMR7YLHy7laTq5yOhjMW+w6rYzncxvyD4=
                got:    sha256-DTspmwyD3Evl1CUmvUy2MonbLGUezvsHN3prmP9eK2I=

Note: it means that some of existing caches for fixed output
derivations become incorrect. It should not break already cached
tarballs on cache.nixos.org thus the impact should not be widespread.

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