Temporarily don't test on stable on CI.

At the time of writing, "stable" is 1.64 on the GHA image, while we need 1.65.

Author: Dirbaio

.github/workflows/test.yml

@@ -20,7 +20,7 @@ jobs:
         # Test on stable, MSRV, and nightly.
         # Failure is permitted on nightly.
         rust:
-          - stable
+          #- stable  # TODO: enable again when "stable" is 1.66 or higher.
           - 1.65.0
           - nightly
 
@@ -64,7 +64,7 @@ jobs:
         # Test on stable, MSRV, and nightly.
         # Failure is permitted on nightly.
         rust:
-          - stable
+          #- stable  # TODO: enable again when "stable" is 1.66 or higher.
           - 1.65.0
           - nightly