narinfo: Change NAR URLs to be addressed on the NAR hash instead of the compressed hash

This change is to simplify [Trustix](https://github.com/tweag/trustix) indexing and makes it possible to reconstruct this URL regardless of the compression used.

In particular this means that 7c2e9ca597/contrib/nix/nar/nar.go (L61-L71) can be removed and only the bits that are required to establish trust needs to be published in the Trustix build logs.

src/libstore/binary-cache-store.cc

@@ -176,11 +176,7 @@ ref<const ValidPathInfo> BinaryCacheStore::addToStoreCommon(
     auto [fileHash, fileSize] = fileHashSink.finish();
     narInfo->fileHash = fileHash;
     narInfo->fileSize = fileSize;
-    narInfo->url = "nar/" + narInfo->fileHash->to_string(Base32, false) + ".nar"
-        + (compression == "xz" ? ".xz" :
-           compression == "bzip2" ? ".bz2" :
-           compression == "br" ? ".br" :
-           "");
+    narInfo->url = "nar/" + info.narHash.to_string(Base32, false) + ".nar";
 
     auto duration = std::chrono::duration_cast<std::chrono::milliseconds>(now2 - now1).count();
     printMsg(lvlTalkative, "copying path '%1%' (%2% bytes, compressed %3$.1f%% in %4% ms) to binary cache",

tests/binary-cache.sh

@@ -55,7 +55,7 @@ basicTests
 # Test whether Nix notices if the NAR doesn't match the hash in the NAR info.
 clearStore
 
-nar=$(ls $cacheDir/nar/*.nar.xz | head -n1)
+nar=$(ls $cacheDir/nar/*.nar | head -n1)
 mv $nar $nar.good
 mkdir -p $TEST_ROOT/empty
 nix-store --dump $TEST_ROOT/empty | xz > $nar