msrv: bump pinned version in CI

I forgot to do this in the previous commit.

Author: BurntSushi

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
         include:
         - build: pinned
           os: ubuntu-latest
-          rust: 1.65.0
+          rust: 1.73.0
         - build: stable
           os: ubuntu-latest
           rust: stable