ios: expose signing keychain to xcodebuild

ios/fastlane/Fastfile

@@ -349,7 +349,11 @@ platform :ios do
       xcode_build_setting("DEVELOPMENT_TEAM", TEAM_ID),
       xcode_build_setting("PROVISIONING_PROFILE_SPECIFIER", PROFILE_SPECIFIER),
       xcode_build_setting("CODE_SIGN_IDENTITY", SIGNING_CERTIFICATE_NAME)
-    ].join(" ")
+    ]
+    if present?(ENV["SYBIL_SIGNING_KEYCHAIN_PATH"])
+      xcode_args << xcode_build_setting("OTHER_CODE_SIGN_FLAGS", "--keychain #{ENV.fetch("SYBIL_SIGNING_KEYCHAIN_PATH")}")
+    end
+    xcode_args = xcode_args.join(" ")
 
     ipa_path = build_app(
       project: PROJECT_FILE,