Don't run cargo update on msrv test

This lets us not break on msrv bumps in our dependencies in keeping with
the policy at https://doc.rust-lang.org/nightly/cargo/reference/rust-version.html
which says "it is expected that each dependency’s version-requirement
supports at least one version compatible with your rust-version.
However, it is not expected that the dependency specification excludes
versions incompatible with your rust-version."

A downside is that committing an updated Cargo.lock may cause msrv
breakage at that time.

.github/workflows/test.yaml

@@ -19,7 +19,6 @@ jobs:
     strategy:
       matrix:
         rust:
-          - $MSRV
           - stable
           - beta
           - nightly
@@ -28,6 +27,8 @@ jobs:
         include:
           - rust: stable
             cargo-update: false
+          - rust: $MSRV
+            cargo-update: false
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master