Normalise CI flags (#2240)

Motivation:

`-require-explicit-sendable` is ineffective without `-Xfrontend` but
combining it with `-warnings-as-errors` surfaces a warning from SwiftPM.

Modifications:

- remove `-require-explicit-sendable`, it's not doing anything
- add `-require-explicit-availability` to nightly jobs

Result:

More consistent CI flags

.github/workflows/main.yml

@@ -13,10 +13,10 @@ jobs:
     with:
       linux_5_9_enabled: false
       linux_5_10_enabled: false
-      linux_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable -Xswiftc -warnings-as-errors -Xswiftc -require-explicit-availability"
-      linux_6_1_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable -Xswiftc -warnings-as-errors -Xswiftc -require-explicit-availability"
-      linux_nightly_next_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
-      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
+      linux_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -warnings-as-errors -Xswiftc -require-explicit-availability"
+      linux_6_1_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -warnings-as-errors -Xswiftc -require-explicit-availability"
+      linux_nightly_next_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-availability"
+      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-availability"
 
   benchmarks:
     name: Benchmarks

.github/workflows/pull_request.yml

@@ -22,10 +22,10 @@ jobs:
     with:
       linux_5_9_enabled: false
       linux_5_10_enabled: false
-      linux_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable -Xswiftc -warnings-as-errors -Xswiftc -require-explicit-availability"
-      linux_6_1_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable -Xswiftc -warnings-as-errors -Xswiftc -require-explicit-availability"
-      linux_nightly_next_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
-      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
+      linux_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -warnings-as-errors -Xswiftc -require-explicit-availability"
+      linux_6_1_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -warnings-as-errors -Xswiftc -require-explicit-availability"
+      linux_nightly_next_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-availability"
+      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-availability"
 
   construct-examples-matrix:
     name: Construct Examples matrix