Allow setting request headers for uploading to Nix caches #657

Currently, the microvm engine can optionally upload built Nix store paths to a configured cache through the nixCache.uploadUrl option. However, it is not able to authenticate against a cache. Some caches, such as attic, require authentication to upload store paths to. Ideally, Spindle should be able to read environment variables for request header information. Say, SPINDLES_NIX_CACHE_UPLOAD_REQUEST_HEADER__AUTHORIZATION or something (double _ used purposefully).