[ci] make build on nightly explicit

Author: sebsto

.github/workflows/pull_request.yml

@@ -29,9 +29,11 @@ jobs:
       linux_5_10_enabled: false
       linux_6_0_enabled: true
       linux_6_1_enabled: true
+      linux_nightly_main_enabled: true
       linux_nightly_next_enabled: true
       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"
       linux_nightly_next_arguments_override: "--explicit-target-dependency-import-check error"
 
   swift-6-language-mode: