Quote MSRV version to avoid float parsing (#11)

Put 1.70 in there (for instance if you want to pin against OnceLock stabilizing) and it will actually test 1.7 as it appears github auto converts this to a float?

Putting in quotes seems to do the right thing here

.github/workflows/check.yml

@@ -79,7 +79,7 @@ jobs:
     # https://docs.github.com/en/actions/learn-github-actions/contexts#context-availability
     strategy:
       matrix:
-        msrv: [1.56.1] # 2021 edition requires 1.56
+        msrv: ["1.56.1"] # 2021 edition requires 1.56
     name: ubuntu / ${{ matrix.msrv }}
     steps:
       - uses: actions/checkout@v3