Delete compressed NARs

Fixes #3891.

src/libstore/binary-cache-store.cc

@@ -153,6 +153,8 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, Source & narSource
 
     auto [fdTemp, fnTemp] = createTempFile();
 
+    AutoDelete autoDelete(fnTemp);
+
     auto now1 = std::chrono::steady_clock::now();
 
     /* Read the NAR simultaneously into a CompressionSink+FileSink (to

src/libutil/util.cc

@@ -494,6 +494,7 @@ std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
 {
     Path tmpl(getEnv("TMPDIR").value_or("/tmp") + "/" + prefix + ".XXXXXX");
     // Strictly speaking, this is UB, but who cares...
+    // FIXME: use O_TMPFILE.
     AutoCloseFD fd(mkstemp((char *) tmpl.c_str()));
     if (!fd)
         throw SysError("creating temporary file '%s'", tmpl);