libstore/binary-cache-store: skip NAR listings if it's not possible to serialize it

Some source trees might not be representable inside of the NAR listing
format v1 as file paths (on Linux) are not guaranteed to be valid UTF-8.

When something like this happens on a large-scale build farm, a
mysterious "queued" but impossible to process job appears, this is
because we cannot write the NAR listing and serialization always fails.

Why did this work before? nlohmann was introduced _after_ such paths
were ingested, see: 09f00dd4d0.

What happened for such previously mis-serialized NAR listings?

```
curl -v 'https://cache.nixos.org/nz8p9hn00r6z7s57581c1hiv39pa1ia6.ls' |
brotli -d | jq .
```

This fixes the build of `sub-batch`
(https://github.com/kl/sub-batch/tree/master/tests/rename_invalid_utf8)
on ForkOS infrastructure.

Many thanks to Puck for the assistance on holding `rr` right on this one
and finding the history of these changes.

Change-Id: I2c2fbac70818e02810f9fd236c3a248187bf5fe7
Signed-off-by: Raito Bezarius <raito@lix.systems>

tests/functional/nar-access.nix

@@ -1,3 +1,4 @@
+{ nonUtf8Inodes ? false }:
 with import ./config.nix;
 
 rec {
@@ -13,6 +14,8 @@ rec {
         touch $out/qux
         mkdir $out/zyx
 
+        ${if nonUtf8Inodes then ''printf "data" > "$out/invalid-\x80file"'' else ""}
+
         cat >$out/foo/data <<EOF
         lasjdöaxnasd
 asdom 12398

tests/functional/nar-access.sh

@@ -2,6 +2,7 @@ source common.sh
 
 echo "building test path"
 storePath="$(nix-build nar-access.nix -A a --no-out-link)"
+strangerStorePath="$(nix-build nar-access.nix -A a --arg nonUtf8Inode true --no-out-link)"
 
 cd "$TEST_ROOT"
 
@@ -79,3 +80,9 @@ rm -rf $cacheDir/nar
 diff -u \
     <(nix store ls --json -R $storePath/foo/bar --store "file://$cacheDir" | jq -S) \
     <(echo '{"narOffset": 368,"type":"regular","size":0}' | jq -S)
+
+# Confirm that there's no more than one `.ls` in the `$cacheDir` because non-UTF8 inodes cannot have `.ls` generated for them.
+[[ $(find $cacheDir -type f -name '*.ls' | wc -l) -eq 1 ]] || (echo "Expected at most one listing file in $cacheDir, found more"; exit -1)
+
+# Confirm that NARs with non-UTF8 inodes can still be listed
+expect 0 nix store ls $strangerStorePath/ --store "file://$cacheDir"