Rename nightly_6_1 params to nightly_next (#332)

Rename nightly_6_1 params to nightly_next; see
apple/swift-nio#3122

Author: rnro

.github/workflows/main.yml

@@ -14,7 +14,7 @@ jobs:
       linux_5_9_arguments_override: "--explicit-target-dependency-import-check error"
       linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_0_arguments_override: "--explicit-target-dependency-import-check error"
-      linux_nightly_6_1_arguments_override: "--explicit-target-dependency-import-check error"
+      linux_nightly_next_arguments_override: "--explicit-target-dependency-import-check error"
       linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
 
   cxx-interop:

.github/workflows/pull_request.yml

@@ -26,13 +26,13 @@ jobs:
       linux_5_9_arguments_override: "--explicit-target-dependency-import-check error"
       linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_0_arguments_override: "--explicit-target-dependency-import-check error"
-      linux_nightly_6_1_arguments_override: "--explicit-target-dependency-import-check error"
+      linux_nightly_next_arguments_override: "--explicit-target-dependency-import-check error"
       linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
       windows_6_0_enabled: true
-      windows_nightly_6_1_enabled: true
+      windows_nightly_next_enabled: true
       windows_nightly_main_enabled: true
       windows_6_0_arguments_override: "--explicit-target-dependency-import-check error"
-      windows_nightly_6_1_arguments_override: "--explicit-target-dependency-import-check error"
+      windows_nightly_next_arguments_override: "--explicit-target-dependency-import-check error"
       windows_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
 
   cxx-interop: