don't close cache manifest file prematurely

ErrorInvalidFormat is not a fatal error so don't close the cache
manifest file right away but instead let cache_final() handle it.

Fixes the following (very common) warning when running the test suite:

    Warning: Unable to write cache file [..]: unexpected seek failure

The seek failure is an lseek() system call that failed with EBADF
because the file descriptor had already been closed.

src/cache_hash.cpp

@@ -437,7 +437,7 @@ Error cache_hit(CacheHash *ch, Buf *out_digest) {
                 return ErrorCacheUnavailable;
             }
         }
-        if (return_code != ErrorNone) {
+        if (return_code != ErrorNone && return_code != ErrorInvalidFormat) {
             os_file_close(&ch->manifest_file);
         }
         return return_code;