ci: Add --skip flags for Swift 6.1 matrix job (#83)

Motivation
----------

Swift NIO's unit test matrix job has been [updated for Swift
6.1](apple/swift-nio@b8c4463),
so additional override flags must be defined.

Modifications
-------------

Define `linux_6_1_arguments_override` for pull requests and the
scheduled build on main.

Result
------

Tests with Swift 6.1 will pass.

Test Plan
---------

Existing tests pass again.

Author: euanh

.github/workflows/main.yml

@@ -15,6 +15,7 @@ jobs:
             linux_5_9_enabled: false
             linux_5_10_enabled: false
             linux_6_0_arguments_override: "--skip SmokeTests --skip TarInteropTests"
+            linux_6_1_arguments_override: "--skip SmokeTests --skip TarInteropTests"
             linux_nightly_6_1_arguments_override: "--skip SmokeTests --skip TarInteropTests"
             linux_nightly_main_arguments_override: "--skip SmokeTests --skip TarInteropTests"
 

.github/workflows/pull_request.yml

@@ -22,6 +22,7 @@ jobs:
             linux_5_9_enabled: false
             linux_5_10_enabled: false
             linux_6_0_arguments_override: "--skip SmokeTests --skip TarInteropTests"
+            linux_6_1_arguments_override: "--skip SmokeTests --skip TarInteropTests"
             linux_nightly_6_1_arguments_override: "--skip SmokeTests --skip TarInteropTests"
             linux_nightly_main_arguments_override: "--skip SmokeTests --skip TarInteropTests"