Use Cargo.lock for testing against MSRV instead of pinning

Some newer versions of transitive dev-dependencies don't work with the
minimum supported Rust version (1.46.0), so I used to pin them in
`Cargo.toml` whenever the MSRV build would break (tedious work). The
disadvantage of that is that older versions of those deps are used even
on newer Rust versions.

The latest breakage is matklad/once_cell#201,
but instead of pinning that to an older version too, this instead
introduces a `Cargo.lock` file that is only used for the MSRV build.

So newer Rust versions will continue to test against latest
dependencies, while the MSRV build should continue to work with the
older versions. One disadvantage with this is that consumers will need
to figure out their own `Cargo.lock` with precise versions that work
with older Rust versions, but that is expected to be a small number -
especially because in this case they are all dev-dependencies anyway.

.github/workflows/ci.yml

@@ -36,6 +36,9 @@ jobs:
           - 1.46.0 # MSRV (minimum supported Rust version)
           - stable
           - beta
+        include:
+          - rust: 1.46.0
+            msrv: true
     steps:
       - name: Checkout sources
         uses: actions/checkout@v2
@@ -47,6 +50,13 @@ jobs:
           profile: minimal
           override: true
 
+      # See https://github.com/matklad/once_cell/issues/201
+      # To test locally, enable the lock file and then run:
+      # $ docker run --rm --user "$(id -u)":"$(id -g)" -v "$PWD":/usr/src/myapp -w /usr/src/myapp rust:1.46.0 cargo test
+      - name: Use Cargo.lock for MSRV
+        if: ${{ matrix.msrv }}
+        run: cp Cargo.lock.msrv Cargo.lock
+
       - name: Run cargo test
         uses: actions-rs/cargo@v1
         with: