CI use 6.1 nightlies (#722)

CI use 6.1 nightlies now that Swift development is happening in the 6.1
branch

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: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
+      linux_nightly_6_1_arguments_override: "--explicit-target-dependency-import-check error"
       linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
 
   integration-test:

.github/workflows/pull_request.yml

@@ -20,7 +20,7 @@ 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: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-            linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
+            linux_nightly_6_1_arguments_override: "--explicit-target-dependency-import-check error"
             linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
 
     integration-test: