Refering to channel: in search path got significantly slower in nix 2.21+

[xzfc]

Previously, it was taking less than 0.1 seconds to refer to a channel, but now it's 5+ seconds.
Might be related: verbose output includes this line: source path '«unknown»/' is uncacheable.

Old (fast)

$ nix --version
nix (Nix) 2.20.6

$ time nix-instantiate -I n=channel:nixos-23.05 -E --eval "<n>"
/nix/store/qinxwzj1pxwg37i5x89kh2m7iap5svqa-source

real    0m0.029s
user    0m0.013s
sys     0m0.011s

$ nix-instantiate -vvvv -I n=channel:nixos-23.05 -E --eval "(import <n> {}).lib.fakeHash" |& ts -s
00:00:00 evaluating file '<nix/derivation-internal.nix>'
00:00:00 performing daemon worker op: 11
00:00:00 acquiring write lock on '/nix/var/nix/temproots/1007652'
00:00:00 performing daemon worker op: 1
00:00:00 using cache entry '{"name":"source","type":"tarball","url":"https://nixos.org/channels/nixos-23.05/nixexprs.tar.xz"}' -> '{"etag":"\"3609b92639bb1b75344c6300d9513105\"","lastModified":1704616576}', '/nix/store/qinxwzj1pxwg37i5x89kh2m7iap5svqa-source'
00:00:00 resolved search path element 'channel:nixos-23.05' to '/nix/store/qinxwzj1pxwg37i5x89kh2m7iap5svqa-source'
00:00:00 /nix/store/qinxwzj1pxwg37i5x89kh2m7iap5svqa-source

New (slow)

$ nix --version
nix (Nix) 2.22.1

$ time nix-instantiate -I n=channel:nixos-23.05 -E --eval "<n>"
/nix/store/qinxwzj1pxwg37i5x89kh2m7iap5svqa-source

real    0m5.354s
user    0m2.459s
sys     0m0.660s

$ nix-instantiate -vvvv -I n=channel:nixos-23.05 -E --eval "(import <n> {}).lib.fakeHash" |& ts -s
00:00:00 evaluating file '<nix/derivation-internal.nix>'
00:00:00 using cache entry '{"_what":"tarballCache","url":"https://nixos.org/channels/nixos-23.05/nixexprs.tar.xz"}' -> '{"etag":"\"3609b92639bb1b75344c6300d9513105\"","lastModified":1704616576,"treeHash":"81fbe527509ebdf4d35a954750efa488ebff2aed"}'
00:00:00 source path '«unknown»/' is uncacheable
00:00:00 copying '«unknown»/' to the store...
00:00:00 performing daemon worker op: 7
00:00:03 acquiring write lock on '/nix/var/nix/temproots/1005446'
00:00:05 resolved search path element 'channel:nixos-23.05' to '/nix/store/qinxwzj1pxwg37i5x89kh2m7iap5svqa-source'
00:00:05 /nix/store/qinxwzj1pxwg37i5x89kh2m7iap5svqa-source

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