Fix CI after uncontrollable dev-dependency update.

Criterion has a transitive dependency that we can't control the version
of, which requires > Rust 1.65.  This means we can no longer run tests
using that version, even though it's our MSRV.  So switch to just
building with our MSRV rather than building *and* testing.  In practice
this should be fine, as building should be enough since we're running
tests with latest stable anyway.

.github/workflows/ci.yml

@@ -16,7 +16,6 @@ jobs:
       matrix:
         toolchain:
           - stable
-          - "1.65"
     runs-on: ubuntu-latest
     steps:
       # Get a checkout and rust toolchain.
@@ -27,7 +26,26 @@ jobs:
           toolchain: ${{matrix.toolchain}}
           override: true
 
-      # Build and test
+      # Build and test.
       - run: cargo +${{matrix.toolchain}} build
       - run: cargo +${{matrix.toolchain}} test
       - run: cargo +${{matrix.toolchain}} bench --no-run
+
+  build:
+    name: Build, MSRV
+    strategy:
+      matrix:
+        toolchain:
+          - "1.65"
+    runs-on: ubuntu-latest
+    steps:
+      # Get a checkout and rust toolchain.
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: ${{matrix.toolchain}}
+          override: true
+
+      # Build.
+      - run: cargo +${{matrix.toolchain}} build