Correctly hijack the file:// uri scheme with _NIX_FORCE_HTTP

Setting the `_NIX_FORCE_HTTP` environment variable is supposed to force `file://` store urls to use the `HttpBinaryCacheStore` implementation rather than the `LocalBinaryCacheStore` one (very useful for testing).
However because of a name mismatch, the `LocalBinaryCacheStore` was still registering the `file` scheme when this variable was set, meaning that the actual store implementation picked up on `file://` uris was dependent on the registration order of the stores (itself dependent on the link order of the object files).

Fix this by making the `LocalBinaryCacheStore` gracefully not register the `file` uri scheme when the variable is set.

src/libstore/local-binary-cache-store.cc

@@ -107,7 +107,7 @@ bool LocalBinaryCacheStore::fileExists(const std::string & path)
 
 std::set<std::string> LocalBinaryCacheStore::uriSchemes()
 {
-    if (getEnv("_NIX_FORCE_HTTP_BINARY_CACHE_STORE") == "1")
+    if (getEnv("_NIX_FORCE_HTTP") == "1")
         return {};
     else
         return {"file"};