libindex: fetcher fails on macos

[BradLugo]

Example:

❯ go test -v -run 'TestFetchSimple/1' ./libindex
=== RUN   TestFetchSimple
=== RUN   TestFetchSimple/1
    fetcher_test.go:33: {Digest:sha256:85443624d9df43f46df8ba7d9c40289bb4701eb57096336dd5b97507ccafc8c7 URI:http://127.0.0.1:59318/0 MediaType:application/vnd.oci.image.layer.v1.tar Headers:map[]}
    time=+809.916µs level=DEBUG source=libindex.(*RemoteFetchArena).fetchFileForCache msg="layer fetch start" arena=&{root:0x14000619c20} layer=sha256:85443624d9df43f46df8ba7d9c40289bb4701eb57096336dd5b97507ccafc8c7 uri=http://127.0.0.1:59318/0
    time=+1.341541ms level=DEBUG source=libindex.(*RemoteFetchArena).fetchFileForCache msg="reported content-type" arena=&{root:0x14000619c20} layer=sha256:85443624d9df43f46df8ba7d9c40289bb4701eb57096336dd5b97507ccafc8c7 uri=http://127.0.0.1:59318/0 content-type=application/vnd.oci.image.layer.v1.tar
    fetcher_test.go:40: fetcher: encountered errors: openat fetcher.fc196ea9: no such file or directory
    fetcher_test.go:43: &{noCopy:{} final:<nil> Hash:sha256:85443624d9df43f46df8ba7d9c40289bb4701eb57096336dd5b97507ccafc8c7 URI:http://127.0.0.1:59318/0 Headers:map[] cleanup:[] sys:<nil> rd:<nil> closed:false init:false}
--- FAIL: TestFetchSimple (0.00s)
    --- FAIL: TestFetchSimple/1 (0.00s)
FAIL
FAIL    github.com/quay/claircore/libindex      0.297s
FAIL

I suspect this was introduced in #1628.

[BradLugo]

Also begs the question if we should have a macos runner.

[hdonnay]

I guess it must have been, but I don't really understand how.
It looks like the arena logging attr needs to be wrapped or stringified for better diagnostics.