CLEANUP: Reduce scope of `header_name` in http_action_store_cache()

This variable is only needed deeply nested in a single location and clang's
static analyzer complains about a dead initialization. Reduce the scope to
satisfy clang and the human that reads the function.
diff --git a/src/cache.c b/src/cache.c
index a0b417e..b7ea562 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -994,7 +994,6 @@
 	struct http_hdr_ctx ctx;
 	size_t hdrs_len = 0;
 	int32_t pos;
-	struct ist header_name = IST_NULL;
 	unsigned int vary_signature = 0;
 
 	/* Don't cache if the response came from a cache */
@@ -1181,7 +1180,7 @@
 		 * future conditional requests to be able to perform ETag
 		 * comparisons. */
 		if (type == HTX_BLK_HDR) {
-			header_name = htx_get_blk_name(htx, blk);
+			struct ist header_name = htx_get_blk_name(htx, blk);
 			if (isteq(header_name, ist("etag"))) {
 				object->etag_length = sz - istlen(header_name);
 				object->etag_offset = sizeof(struct cache_entry) + b_data(&trash) - sz + istlen(header_name);