add github_access_token option

I'm starting to see rate limit issues with GitHub Actions more
frequently so I propose to make this option more readily available.

install-nix.sh

@@ -21,6 +21,10 @@ add_config() {
 add_config "max-jobs = auto"
 # Allow binary caches for user
 add_config "trusted-users = root $USER"
+# Add github access token
+if [[ $INPUT_GITHUB_ACCESS_TOKEN != "" ]]; then
+  add_config "access-tokens" "github.com=$INPUT_GITHUB_ACCESS_TOKEN"
+fi
 # Append extra nix configuration if provided
 if [[ $INPUT_EXTRA_NIX_CONFIG != "" ]]; then
   add_config "$INPUT_EXTRA_NIX_CONFIG"